Is 7.5 hours long enough for a good day at Six Flags?

Yes. I have been to six flags for only about three hours and I have been able to enjoy it. You should be able to get in a good share of games and rides. Depending on the places you eat, you could get more out of it than expected. But really, seven hours at six flags is just about perfect. If you go longer than that, you could get really tired.