We were just down there. They do post the return time at the Fastpass distribution area. They also have printed on your Fastpass when you will be able to get another one, so this takes the "guessing" out of when a machine will accept your ticket.
Now, the people who go to WDW a lot have some idea of what rides are slow loading, etc. If I am at Fantasyland in the MK, I know that IASM is ALWAYS a walk-on later in the day, so I don't get in line. I know that Dumbo and Peter Pan are slow loading, so these would be rides I would be inclined to get a Fastpass for as the standby line is EXTREMELY slow so will require a lengthy wait. Bottom line, I am selective about which rides I get my Fastpass at. I would never wait in a long line for Pirates or for the Jungle Cruise ... these rides always seem to have little or no wait periodically throughout the day, so we'll just come back if the wait is too long.
Many of the more popular rides can be done with little wait early in the day ... this necessitates you being there at the time the park opens.
If you are new to WDW, I would encourage you to read the Unofficial Guide to WDW. I think they do a good job in explaining some of these concepts and describing which rides have lines that move slowly (slow loading rides) and which move quickly (rides that can accomodate more people in a shorter amount of time).
As with most things in life, a little time spent in preparation can save a lot of time and frustration later on.