It depends on the student, and the structure of the course.
For the "typical" PPL student, only flying at weekends, 15 hours is a reasonable time. The better continuity the student has, the earlier he should go solo. A full time student should be looking at solo at around the eleven hour stage.
Another factor is age. In general, younger students should solo earlier. Also, if they have gone through some kind of aptitude test (successfully of course) this should reduce still further, possibly to the 8-9 hour margin.
At the risk of attracting the label of male chauvinist, I have found that female students tend to take a little longer to solo, but catch up later (by around the nav stage).
To sum up: 8-20 hours. Clear as Mud!