Carl Bialik from the Wall Street Journal made an interesting analysis of the time players take inbetween serves, which is often longer than 20 seconds ("a maximum of twenty (20) seconds shall elapse from the moment the ball goes out of play at the end of the point until the time the ball is struck for the first serve of the next point"). Players do not really pay attention to the rule, probably because it is not enforced much.
Here’s the average among 20 women timed by stopwatch at the US Open: 22.7 seconds and 12.2 pre-serve bounces of the ball. Interestingly, women’s tennis players need shorter breaks before serving after they have won a point – average of 21.6 seconds and 11.2 bounces. However, they need longer preparation after losing a point – average of 22.8 seconds and 12.5 bounces.
And one more thing, even though the second serve should follow an unsuccessful first serve "without delay", female tennis players take 11.5 seconds and 6.2 bounces. (via: Women Who Serve, photo: © Neal Trousdale)