Once you receive an acceptance letter, you will have two weeks to respond in writing to request that a spot be held for you. While not all universities mention this two week window, if you do not reply in two weeks, your offer will be rescinded. I have heard several stories of students who wait more than the allotted time, only to lose their only shot at going to a particular school. In most cases, the decision was final. If the offer is rescinded because of a lack of timeliness on your behalf, few colleges will even allow you to petition the decision.
Generally, you will have to pay a deposit when you request that a spot be held for you. This amount can be anywhere from $30 to up to $500. In many cases, these deposits are non-refundable. Therefore, you should only hold a spot if you a genuinely interested in attending the school. If you were turned off during the campus tour and interview, don't hesitate to withdraw your application, especially if you have multiple acceptances. Keep in mind that for every spot you hold, you keep someone else from getting an acceptance. As a courtesy to others, relinquish the schools that you are no longer interested in.