Having checked my ancient logbook (Vol 1) I can report a total of 4.8 hrs of circuits before First Solo. At some 6-7 mins per circuit this comes to .. err .. umm .. say 45 ? Bit of a guess really.
Speaking as an (ex-)instructor, I would let them loose after they no longer scared me and demonstrated a sporting chance of return to earth without damage to anything or anyone and an appropriate level of self-confidence. This could be anything from a minimum of about 4 hours to one very determined, patient and eventually successful lady in her 60s at about 20 hours on the circuit.
I agree that, in general, the younger you are the quicker you will learn. But, cost aside, it is not a race so enjoy it. I loved doing circuits while learning and I still do after 30+ years flying.
SI