I don't know if there is any proven trend in 'hours to solo' and its all pretty personal.
Where I did my PPL it is not uncommon (though not the norm either) for younger students to go solo after 5-10 hours instruction. Comparing this to the next closest school where the same type of student will probably take closer to 20 I would say the biggest factor is the airport/location.
In the first it is an uncontrolled field, with lots of airspace around it but little to get in the way of doing exercises. Then when it comes to circuits, on a quiet day you can do plenty in an hour. Looking through my logbook I see one hour block time where we did 12 circuits.
At the other place it is at a controlled airport with scheduled traffic which means that much more time is going to be spent to complete the same amount of instruction.
Edit just to add that personally I went solo after 16.8 hours. This included 1 hour of 'trial flight' that was 2 years prior to starting lessons and it took over 6 months to do because I was still doing my A levels and could only afford occasional lessons. It also was across 4 aircraft types (for numerous reasons). The reason I am adding this is to show some of the personal factors that will change it. Could I have gone solo in less time if I did all the lessons in one week and stuck to one aircraft type and didn't have school exams at the same time? Maybe - but it's irrelevant because it was not the case.