These days, most pilots solo with between 15-20 hours, and more towards the 20 hour mark. As with everything pertaining to flight instruction, YMMV. I solo'd with just under 19 hours. I think AOPA did a survey recently and the average number of hours prior to solo was 20.x or something. At my little field in the past two years (my flying time), I haven't seen anyone solo with fewer than 17 hours and most are in the 21-23 hour range.