varies from school to school, but it seems the usual response time is within two weeks with a deposit. Having one acceptance does NOT preclude you from going to other interviews or accepting places at other schools. You can withdraw after you accept, usually with a refund of your deposit money, until May 15 I think. After May 15 you forfeit your deposit money but can still withdraw from one school and accept another (for example getting off a waitlist).
I don't know about extensions.