At a professional tournament, statistics were taken for the length of the rallies.  These statistics are fairly common across the board for all levels but differ based on game-style.
 
On average…
12% of serves are not returned
18% of points are won on the 2nd ball
26% of points are won on the 3rd ball
13% of points are won on the 4th ball
11% of points are won on the 5th ball
6% of points are won on the 6th ball
4% of points are won on the 7th ball
10% of points are won after the 7th ball