I would say the average is 15 hours with a standard deviation of 5 hours. Which basically means that most, but not all, do it after 10 to 20 hours.
This question has been asked before a number of times here. A lot of people posted their numbers plus any factors (prior gliding experience for instance) that may have influenced it. A search plus some statistical analysis might help narrow my guesstimates down.
But as others have said: once you've got that PPL, nobody cares anymore.