I would have said that the average in my experience of students I sent solo, was between 20-25 hours, obviously this depended on my philosophy as well as there aptitude etc etc,
I never sent someone in less than 20 until I had one guy who was remarkable, everything you showed him he reproduced perfectly first time. he went solo in 9 hours and we had covered all exercises from 4-11, plus 15 & 16
I wouldnt have believed it was possible, but I had more confidence in him with 9 than some other people that took 30-35.
so a lot depends on the student, much more than you would think in my opinion
regards
CF