Your flight time will depend on capacity of battery, type of motor (size and current draw), weight of plane including wing loading, and flying style.
Take my Mini IFO for instance. I'm flying it with an IPS-A and a 2cell 950mah Li-poly. I get about 15 minutes of aerobatic flight before power really fades. Now if I put on a 2cell 2100mah batt, I could get much more flight time, but the plane wouldn't be near as fun because the 2100's weigh much more than the 950's. The plane would require more throttle to maintain flight, and all manuvers would also require more throttle. It's a balancing act. Generally with an IPS powered plane, I'd stick to a smaller pack to keep it light. Besides, 15 minutes of aerobatics, is about all I can handle.