Why does your antifreeze run out so quick?

It might be a freeze plug open on the motor (a safety feature). Once the level gets low enough, it will stop leaking.

ANS 2 - More likely is that it's leaking while the engine is running and hot. Check at the base of waterpump. Also any hoses to heater,etc. If you don't see any leaks and it still leaks it may be the head gasket. ( Let's hope not.)