This is really school dependant. Some schools require you to respond to their acceptance within 2 weeks (many schools are like this, actually) while others have no official deadline. The school to which I was accepted has no deadline, but I still sent it in within a week.
Many schools require you to submit a deposit in order to hold your place in their class, usually its $100 or so and refundable up until May 15th. This by no means restricts you to only attend that school, and you can hold multiple acceptances to different schools. However, after May 15th you are only allowed to hold one acceptance at a time and are required to withdraw your application from all other schools. If you are accepted to another school after May 15th and wish to attend there, you can withdraw your application from your current school and switch to the new one.
Or you could have used the search function instead of making me type this 🙄