Some instructors will send students solo when they can just about manage to fly a circuit if all goes well. Others like to make sure they'd be able to cope with engine failure after take-off, complicated ATC instructions if things are crowded, or a diversion to another airfield if someone crashes on the runway (it's happened!). Most are somewhere between those two extremes.
Add to that different weather conditions, runway lengths and widths, frequency of flight, and type of aircraft, and you have a whole load of variables before you even consider student age, aptitude etc.
So there is no average. And if there were, if would be meaningless.