I have flown my NexStar for as long as about 15 minutes. I wanted to see how long it would fly (not full throttle) and also wanted to practice a deadstick.
The problem was that I was the only at the field and was flying pretty high up. By the time I landed I lost track of how long I glided hahaha. But I would guess that I flew close to 15 min with the engine running. What I do now is set my timer for 12 minutes and land when it goes off. I may have to do a few go-arounds so that gives me some time.
One time I wanted to fly full-throttle for most of the flight so I adjusted my timer accordingly - set it for 10 minutes.