I wouldn't worry about how long it takes you to solo. Everyone learns at thier own pace. Some catch on almost imediately, others take years. For an average, I'd say 20 flying sessions. Taking over only three times on your 13 minute first flight is good, but to solo you need to be at the point where the instructor didn't need to take over once in 13 flights. Keep working on both the sim and the real deal and you will solo soon. Cheers.