The original poster answered his own question, there are simply too many variables to provide a definitive estimate. Consider student aptitude, instructor experience and skill, the chemistry of an individual student/instructor relationship and the permutations are endless. An important factor, of course, is continuity of training. All else being equal, someone on a full-time course, flying most days will progress more quickly than the student who only flies occasionally. The important thing to remember is that time/number of circuits to solo has little bearing on the eventual competence of the licenced pilot who emerges from the training.