The machine does tell you the return time before you get your ticket.
In April after easter, it was always for an hour later. Sometimes that was too soon and we wished the return window started later.
We always got to the park before it opened, grabbed a Fastpass for a major attraction, and rode the other major attractions in that area, grabbed another fastpass and then rode our fastpass ride.
Or, we'd ride the FP ride and then grab another FP when we got off....it all depends on where you are and where you are headed.
They can be really spread out, and it's a lot of running.
It saved us hours of waiting in line, and we were able to ride all our favorites twice using FP.
Karen