For the most part, it's as a previous poster said: either 2 hours later, or after the return time on the first fastpass, whichever comes first. However, I read in the "Unofficial Guide" that one exception is the Tower of Terror, where you can get the next fastpass an hour later. Other than that, while there is no officially set limit on how many you can have at once, the way the system works, generally will prevent you from having more than two or three at a time, barring the random surprise fastpasses that may pop out. I'm not saying that things couldn't fall into place in certain situations such that you might have more, but for the most part, I think two or three is the most you're gonna have at once. At least current fastpasses. We won't get into holding onto expired fastpasses and using them later, since the CMs tend to overlook it. lol
