I did say "say" 15 hours.
Derived from recent experience at a busy GA airfield where they have a lot of foreign students. They were expected to solo at 20 hours and if they did not, were subject to various reviews before being allowed to continue. Given that these guys had very poor English and little prior exposure to any mechanical transport beyond a bicycle, my 15 hour estimate is what I would expect from a westerner who had at least driven a car.
Way back, solo was typically 6 to 8 hours, but back then we did not have much traffic or radio to worry about.
Certainly, continuity and quality of instruction is important - hence my qualification with the word "say", but any school which is not getting students off solo at about that average point is probably ripping them off or is itself sub-standard.