I always make my earliest fast passes sometime around 10:45. Going at rope drop, you can ride several things without much of a wait at all. I figure by 10:30-11:00 lines have started to grow. Some people say they have lots of luck getting their passes early and getting to choose a 3rd, 4th, 5th....etc. Other people get theirs in the early afternoon and later and just wait in standby lines until later in the day. Personally I like to spread mine out a little (10:45, 1:30, 4:00). It gives me time to do things like lunch and rides that might not have too much of a wait without worrying about being in line and cutting it close to my fast pass window. I have been able to add later fast passes for a couple of the headliners before and there have been times when there was nothing we wanted to do left. I never worry too much about getting extra fast passes, but I also don't really mind waiting in line if it is something I really want to do.
I remember going as a young teenager when there were no fast passes (pre 1999) and waiting over two hours for many of the rides. It was just what you did.