As most of the folk here have already said, its whenever your intructor feels confident enough to send you on your way and when you feel ready. You'll get to the point where your circuits are almost faultless and you'll know its time!
I did 3 hours of circuits before I was allowed to go solo. Infact before I was allowed to go, I had to fly with the CFI and do a couple of circuits with him. I went solo at 13.4 hours... that was a month ago now! Just about to do my local area solo
