I can't help but feel that a good instructor will know when the time is right for you, the student to take that step. Trust his or her judgement. He/she won't let you loose if he/she doesn't believe you can do it. The paperwork that follows would be just too much hassle. You, the student, will probably also have some idea that the time is about right too, and like that first time you headed off on your bike without the stabilisers or a caring hand on the saddle, it has to be done. If you're not prepared to trust your instructor's judgement, see the sentence at the end of the next paragraph.
Whilst there may not be a particular "right" number of hours, there may (must/should) be a point at which either student or instructor will recognise that this just isn't working out. That may be the time to call it quits and go and do something else and, just out of curiosity, how goes the bike riding?
I was somewhat taken aback to be discussing hours with a young chap and his dad recently to find that the lad had done 25+ hours and was still not off by himself. Perhaps the lack of an air law pass was affecting things, but hey.... come on.
From the many books written by heroic aviators like Bader, Stanford-Tuck, Oxspring and, perhaps also the not-so-heroic, and the like I would guesstimate that around 8-12 hours is about the norm for a first solo, and often it is better to have it sprung upon you at the time rather than having a couple of nights to dwell upon the prospect. If it affects you that badly, then refer to the last sentence of the second paragraph
My Dad, a 20 year old RAFVR cadet pilot (young whippersnapper) soloed on Tiger Moth T5842 on 6th April 1944 at RAF Shellingford after 9hrs 35mins dual in 18 flights
His KOS of a son (a bit over 50 years old at the time) managed it at in 11hrs (Dad's still one up on me there), after 9 flights in a PA38, at Sleap in 2003, and I would have to say that I was surprised and delighted when told to go off and do a circuit by myself. I trusted my instructor's judgement, and maybe mine. Thanks Alan.
As with passing your driving test, you learn more when you are in sole command, so it is with this flying malarky. The first solo is a milestone and just another point from which more learning develops. Every time I fly I learn; whether with an instructor, or solo or with passengers. Recognise that you cannot ever know it all at any particular point in time, but if you want to learn and develop your skills and experience you will.
Octavian