As others have mentioned, the average is not all that useful, since the range of different people is so vast, but the number I believe is 4 gallons of fuel, or about 60 flights. However, I define solo a little differently than some. I'm talking about real competence and confidence with a trainer-type airplane. You can fly in a mild cross-wind, you can put it down exactly where you intend to, you can land either direction, you can recover from a dead stick on take off, etc.

If you define "solo" as the first time you successfully take off, fly around, and land by yourself (i.e. the guy with the buddy box never has to take control), then I would guess the 15 number is about right.