This really is one of those "piece of string" questions, I have known students go solo at under 5 hours (very rare), I also took over a (not yet solo) student in my early days of instructing who already had over 50 hours and must have done at least 200 circuits, he had been recommended by at least 3 instructors to pack it in but continued because he enjoyed it, I took him out of the circuit and we just had some fun then did 3 circuits at the end of each detail, he finally went solo at about 60 hours having done probably another 10-15 circuits and eventually got his PPL.
