They say it 's about fifteen to twenty hours, but then again it also depends on your instructor. I had one instructor who was keen to let me go at about twelve because he was confident that I could fly but he retired [instructed for twenty years] before I got there . My next instructor was a lot less experienced and therefore less confident in his own decisions, so with him making the decision I went at seventeen hours.
I do however recommend saying ALL your checks out so the instructor can hear them, that way he/she will know that you know whats going on and let you up solo maybe a little earlier. Just enjoy it though
