it depends on the plane, engine, prop, nitro content, and how heavy you are on the throttle.
he longest ive flown on 8oz is 47min and 15 sec with a gp rapture (average sports plane, not a glider) that was w/ a gms .47, wildcat 10% nitro and 11x5 apc prop
for the most part if your are flying in circles, 20+ min. aeorbatics 15 min, wide open throttle 10 min.