The biggest limit is normally the engine fuel and oil system. Most engines will last about 15 secs with the oil that is already circulating but after that the bearings get starved as the oil in the tank gets lifted off the inlet. Aerobatic display aircraft have a special oil system, and possibly fuel system as well, to ensure continued supply when inverted.
Aerodynamically speaking the wing doesn't need to be cambered. You can fly a flat plate - the problem is that it would have no strength and would bend up, not to mention having a rather sudden stall.