Average number of games in World Series, 1995-2012 (LDS/LCS era, best-of-five/best-of-seven): 5.33.
Average number of games in World Series, 1985-1994 (LCS era, best-of-seven): 5.89.
Average number of games in World Series, 1969-1984 (LCS era, best-of-five): 5.94.
Average number of games in World Series, 1922-1968 (WS only, best-of-seven): 5.83.
The trend appears to be, the more series you play, the shorter the World Series will be.