The time stamp for next fast pass is a complicated formula based on crowd size and ride popularity and "traffic" patterns.... You can hold many FP's at the same time because while they stagger your ability to get them you can hold them all day if you want. They are always valid, even after the return window is up. Also some machines are 'disconnected' or on a seperate system, so you can get these ones even if you just got another one. Look for disconnect posts near the time you are going. Also from time to time Disney plays with the system to try out diff. senerios (like 3 FP's at a time, unlimited, etc.). I have heard that in CA there is a special ticket that lets you get unlimited FP's, but I don't remember how...