On busy days, Fast Passes can be issued for times hours ahead. In such cases, when are the next passes available?
If I were at MK and it was so busy that (for instance) Splash Mountain FPs were issued 5 hours ahead, I might be inclined to get the heck out of there and try a different park. I suppose this could put you in that situation./QUOTE]
Maximum time between fast passes is about two hours, sometimes less during less busy days.
Occasionally you can get a fast pass for a specific ride well in advance of the time for next fast pass shown on the fast pass you just got. This is not advertised. Example: Lights Motors at MGM allows getting a fast pass about 5 minutes before or after getting your previous fast pass.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm