I'm an instructor in probably one of the busiest airspaces in the world, and see student pilots soloing at 10 - 20 hours. I really think it has to do with the attitude of the instructor, along with how hard the student is working towards his/her license.
I have soloed a guy in 25 hours because he just wasn't comfortable when i felt he should have been solo, and I soloed a girl (that was being pushed through in 50tt hours), in 12 and she still turned out a very competant pilot.
Yesterday I soloed a guy who was doing it purely for fun in 10 hours, in which i had plenty of time to cover and review all the lessons required by the US regulations.
I myself was soloed in 6 hours glider, and 7.5 hours airplane at 16, and 18 years of age...Its a really hard question to answer i guess its best answered by whoever wants to go solo.
I think as an Instructor with a bit of experience, it's when my student elects to go-around after I don't say anything during one or two patterns/circuits.