You either have an actual fuel leak somewhere or more likely the drainback valve for the fuel system is faulty (it's generally in or near the pump depending on the vehicle model). Your fuel rail is slowly leaking back into the lines and tank instead of remaining primed with fuel. It's generally a slow drain which is why it has to sit for a while for it to happen. The multiple cranks needed are actually re-priming the system (getting fuel into the injector rail) before it will fire up.
Alternatively you might have a leaky injector (or several) which is causing the same symptom. However, given how long you need to crank it that by itself is unlikely as it sounds like the fuel rail is pretty empty--that much fuel in a cylinder or two would be...unpleasant to say the least.
