When I get a FP, I always look at the information under the return time to find out when I can get another FP.
I just pulled out a few old FPs from past trips.
One was picked up at 9:20pm with a return time of 9:50pm-10:50pm(RNRC). It says I could pick up another FP at 9:55pm
I have one from Soaring that was picked up at 4:28, return 5:05-6:05. That one says " YOu can obtain additional FastPass tickets NOW, at any other FastPass location" (Spring break 2005)
Buzz picked up at 8:48am. Return time 10:05am-11:05am. Another FP can be picked up at 8:54am(This one is from Good Friday 2004)
I've always heard that you had to wait 2 hours between FPs. However, that doesn't seem to be the case most of the time.