It is free, and here are some of the rules. When you get a fastpass, it tells you when to come back and this is called your return window. Your return window is for an hour. Now, I believe it is five minutes AFTER your return window OPENS, you can get another fastpass. So, the key is to get a fastpass with a quick return time. For instance, I was just at MK on thursday, and I went to tomorrowland and got a fastpass for Buzz with a return time that was thirty minutes away, so after I got the FP, I rode TTA and COP, then when my return window opened, I went to Space Mountain before riding Buzz, this way my time on Buzz was eating up time towards Space Mountain. So my key is get a FP with a fast return time, and you will be able to get more fastpasses, because if your return window is two hours or more away, then the longest you will have to wait to get another FP is two hours.