Schools usually post a deadline by when you have to make decisions. Additionally, if you are applying in R1, some schools require you to pay a non-refundable deposit that is deducted from the tuition cost once you decide to enroll.
Building on top of mba_it_is' excellent answer, you will probably have to make your deposit some time in march if you are a typical R1 applicant. Early decision applicants are required to deposit mostly in january.
You can check the universities' websites. There is usually that info in the "deadlines" or "important dates" pages.