We have been able to get a FastPass about every hour or so. It tells you on your ticket when you can get another one… When we first arrive at MK early a.m., we go straight to Space Mt. get a FastPass, then either ride it without one (if the line isn’t too long) or go to Splash Mt., ride and then go to use our Space Mt. FastPass.
At MGM, The FastPass machines for ToT and RnR are close together, so we always grab a FastPass for one and then ride the other.
DS wants to make sure we do the same for Test Track & Mission:Space this next trip (honestly, I have never seen the attraction to Test Track - the drive down to WDW is usually enough automobile excitement for me! - lol)