Each ticket gets put the machine separately and out pops out a little piece of paper along with the ticket with an hour long return time. The return time is posted there. Depending on the popularity of the ride is how fast the fast passes run out... For example, in soarin, test track, and toy story mania, the fast passes go very quickly, and often run out around noon. So, try to get one of those first thing. You can only get one fast pass per ticket, but it will also say what time you can get another fast pass...
Pretty complicated explanation for something quite simple... But once you get the hang of it....