It took me about 9.5 hours (tach time in a Chief) to solo, never seeing pavement or an electrical system. I've had students solo in under 10 hours, but I've also had one that took over 30 hours. So it varies a LOT, depending on the student, how well the student and the CFI mesh, how often the student flies (more often == less time spent on review), as well as weather and other factors.