Circumstances dictate time to solo. I was an airfield rat and given many opportunities prior to being legally old enough, six hours ten minutes to solo in a Chipmunk when of age. Joining the military it was 15.7 hours to solo, because the syllabus said the solo would be made on the thirteenth flight should there be no hiccups - failure on a trip, interruption due mechanical, weather etc 11.8 hours to solo a helicopter, the syllabus once again saying you went solo on the eleventh flight. Continuity is important as well, lengthy intervals between trips involves reviewing previously flown exercises. In my early civil days eight hours was the generally accepted norm at my club if you flew every weekend, a time when all students were ultra keen, the married men with families had pooled resources to work second jobs on the weekends digging septic pits to earn their flying money.