There was a rally in a match at richmond in october 1984 between Vicky Nelson and Jean Hepner which lasted 29 minutes(643 strokes)!!!
The number of strokes is clearly exagerated. That corresponds to a stroke every 3 seconds. Those girls were moonballing, probably only striking the ball every 7 or 8 seconds.
If you calculate at 8 strokes per minute its around 250 strokes for 29 minutes.