To better make examples of my point is to use some contemporary MLB managers. Terry Francona's first game as a manager was in 1997 with the Philadelphia Phillies. He is active through the 2014 season. That covers 18 years, not just the 14 that he has managed. Baltimore's Buck Showalter managed his first MLB game in 1992, so he has covered 23 seasons, 14 of them behind the bench. Mets manager Terry Collins has 21 seasons between his first and last game as a manager, with 10 seasons as a manager. Seattle's Lloyd McClendon first managed a game in 2001 and managed this season, so he has 14 seasons between his first and last game, with 6 seasons as a manager. Hall of Famer Babe Herman managed his first game in 1947 and his last in 1966. Though it covered 20 seasons, he only managed for 4 of the years.
All of those I have looked up had at least 28 years between their first and last game as a manager. Some are obvious while others benefited from a little time off.
#15 (6 are tied)-
Bobby Valentine- Bobby started out as the Rangers manager from 1986-1992. He would start managing the Mets in 1996 and stayed there through the 2002 season. His last stop was with the Red Sox in 2012. Overall, his span covered 28 years from 1986-2012. However, he was only behind the bench for 16 of them, not including his time in Japan.
Roger Peckinpaugh- Peckinpaugh, at age 24, was the youngest player manager in the history of MLB. He managed the New York Yankees the last 20 games of the 1914 season. He would not return, waiting until 1928 when he became the manager of the Cleveland Indians, a job he would hold through 1933. He came back to manage the Indians again in 1941. Though the span from his first to last game was 28 seasons, he only managed in 8 of them.
John McNamara- John started out managing the 1969 and 1970 Oakland Athletics. He then took over the 1974-1977 San Diego Padres, the 1979-1982 Cincinnati Reds, the 1983-1984 California Angels and the 1985-1988 Boston Red Sox. He finished off by managing the 1990 and 1991 Cleveland Indians before finishing off in 1993 with the Angels again. His career covered 28 seasons, with 19 of them behind the bench.
Gene Mauch- Gene managed consecutively from the years of 1960-1982, 1960-1968 with the Philadelphia Phillies, 1969-1975 with the Montreal Expos, 1976-1980 with the Minnesota Twins and 1981-1982 with the California Angels. He returned to manage the Angels from 1985-1988. From start to finish, his managerial career covered 28 seasons, 26 of them behind the bench.
Jim Leyland- Jim managed the Pittsburgh Pirates from 1986-1996, then the Florida Marlins from 1997-1998 and the Colorado Rockies in 1999. He returned to manage the Detroit Tigers from 2006-2013. 28 seasons, 22 of them as a MLB manager.
Del Baker- Del started out by managing the Detroit Tigers in 1933, then managed them again on separate occasions in 1936 and 1937. He then managed the Tigers from 1938-1944. His last stop was in 1960 for the Boston Red Sox. His 28 year career covered just 9 seasons as a big league manager.
Charlie Grimm- The Chicago Cubs manager of their last World Series appearance started out with the Cubs in 1932, staying there through the 1938 season. He returned to manage the Cubs in 1944, where he remained through the 1948 season. After managing the Boston and Milwaukee Braves from 1952-1956, he came back one more time to manage in Chicago for the 1960 season. From start to finish, Grimm covered 29 years of major league baseball, with 19 of them as a manager.
#12 (2 are tied)-
Wilbert Robinson- Wilbert was the player/ manager for the last of the first Baltimore Orioles team in the AL in 1902. He took over the Brooklyn Superbas for the 1914 season, with the name changing to the Robins- off of Robinson's last name. He stayed there through the 1931 season. Behind the bench, Wilbert spanned 30 seasons, 22 of them as a manager.
Davey Johnson- Davey took over the New York Mets and remained there from 1984-1990. His next stop was in Cincinnati with the Reds from 1993-1995. He then managed the Baltimore Orioles from 1996-1997. After a year off, he skippered the Los Angeles Dodgers in 1999 and 2000. He then took over the Washington Nationals from 2011-2013. From start to finish, it was 30 seasons, with 17 of them as a manager.
#9 (3 are tied)-
Frank Robinson- The Hall of Fame OF got his first shot as a big league manager from 1975-1977 with the Cleveland Indians. His next stop was in San Francisco with the Giants from 1981-1984. Then, he landed with the Baltimore Orioles from 1988-1991. Finally, he managed the Montreal Expos/ Washington Nationals from 2002-2006. Overall, he covered 32 seasons, with 16 of them as a manager.
Casey Stengel- Casey's illustrious managerial career started out with the Brooklyn Dodgers from 1934-1936, then moved on to the Boston Bees and Braves from 1938-1942 and again for the second half of the 1943 season. He then managed the New York Yankees from 1949-1960 before finishing off with the New York Mets from 1962-1965. His span covered 32 seasons with 25 as a manager.
Bill McKechnie- Bill started his managerial career off in the Frontier League for the Newark Pepper in 1915. He then managed the Pittsburgh Pirates from 1922-1926, then the St Louis Cardinals on two separate occasions in 1928 and 1929. He then managed the Boston Braves and Bees from 1930-1937 before finishing off with the Cincinnati Reds from 1938-1946. He spanned 32 seasons with 25 of them behind the bench.
#5 (4 are tied)-
Tony LaRussa- LaRussa is the only manager on this list to have one consecutive term as a manager without any interruptions. He started out with the Chicago White Sox from 1978-1986 before taking over the Oakland Athletics from 1986-1995. He then managed the St Louis Cardinals from 1996-2011. He managed for 33 seasons, the entire 33 years of his span from start to finish as a manager.
Bucky Harris- Bucky started out his managerial career as a player/ manager with the World Champion 1924 Washington Senators. He stayed there from 1924-1928 before managing the Detroit Tigers from 1929-1933. He then managed the Boston Red Sox in 1934. He then returned to Washington where stayed from 1935-1942. He then managed the Philadelphia Phillies in 1943 before taking a break. He signed on to lead the New York Yankees to a World Series Championship in 1947 and was let go after the 1948 season. He then started his third run as Senators manager from 1950-1954 before returning to Detroit for 1955 and 1956. His 33 year span included 29 years as a manager.
Chuck Dressen- Chuck started out as manager of the Cincinnati Reds from 1934-1937. He did not manage again until 1951 when he led the Brooklyn Dodgers where he remained through the 1953 season. He then led the Washington Senators from 1955-1957 before leading the Milwaukee Braves 1960-1961. He concluded his career in Detroit, with two separate stints with the Tigers from 1963-1964 and 1965-1966. His 33 year span as a big league manager covered just 16 seasons.
Bobby Cox- Cox managed his first game for the 1978 Atlanta Braves, where he would remain through the 1981 season. He then took over the Toronto Blue Jays, where he stayed through the 1985 season. His return to the Braves started in 1990 and he managed through the 2010 season. From start to finish, Cox's managerial career covered 33 seasons, 29 of them as a big league manager.
#3 (2 are tied)-
Joe Torre- Torre started his career as a player/ manager for the New York Mets in 1977. He remained through the 1981 season. He then managed the Atlanta Braves from 1982-1984. He then took over the St Louis Cardinals from 1990-1995. Torre would then manage the New York Yankees from 1996-2007 and finished off behind the bench for the Los Angeles Dodgers from 2008-2010. His career spanned 34 years, 29 of them as a manager.
John McGraw- McGraw started out as a player/ manager for the NL's Baltimore Orioles in 1899. He then skippered the AL's Baltimore Orioles in 1901 and 1902. He left the Orioles to take over the NL's New York Giants where he would remain through the 1927 season. He returned in 1928 and stayed through the 1932 season. McGraw's 34 years spanned his 33 seasons as a big league manager.
Leo Durocher- Durocher started out as player/ manager for the Brooklyn Dodgers from 1939-1946. After being suspended for the 1947 season, he returned to lead the Dodgers in 1948. After he was fired in 1948, he took over the New York Giants and managed them from 1948-1955. After some time off, he managed the Chicago Cubs from 1966-1972. After he was let go by the Cubs in 1972, he managed the Houston Astros from 1972-1973. His run spanned 35 seasons, with 24 of them behind the bench.
Jack McKeon- McKeon started out managing the 1973-1975 Kansas City Royals, then led the 1977 and 1978 Oakland Athletics in separate stints. He then led the San Diego Padres from 1988-1990 before leading the Cincinnati Reds from 1997-2000. He then was named manager of the Florida Marlins from 2003-2005. He finished off his run with the Marlins in 2011- at age 80. Even though he spanned 39 seasons as a big league manager from start to finish, he was only a manager for 16 of them.