My own experience suggested that it depends very much on the intention of the instructor. I started my ppl training about 3 years ago with a prepaid 45 hr package course.
By the time I did 43 hr total and about 140 touch and go, there was still no suggestion from the full time instructor that I could go solo . When I politely asked when I could do it, he just frown and said I needed to do the circuit and landing perfectly befor I ccould go solo. I felt, as a low hour student, I could not do perfect landing, so I discussed briefly with the CFI and he didn't seem to suggest anything else I could do. Out of despereration, I changed to a part time instructor, who was a retired gentleman, and within one and half hr flying he was happy to send me solo, and I did the best landing I've ever had. The full time instructor left for an airline job soon after.
Hope my experience contribute to the discussion on this subject.