As many have said, it's about being ready more than anything else. Not to mention that for a first solo to happen, on the day you need a combination of factors in your favour too - calm winds, good vis, happy instructor, uncluttered circuit pattern, etc etc. You could end up doing an extra hour or two of dual circuits even after you are technically competent, just because you needed to wait for the "right conditions". I did my first solo after 18 hours, and I flew part-time at one of the real schools, not the sausage factories.
I think you the more frequently you train, the faster you become competent... especially during the pre-GFPT phase, where it is all about learning the skill of aircraft handling.