Most rides will take less than 10 minutes if they’re running smoothly. There are exceptions, like Indiana Jones has a lot of queue left after the FP line joins up, so that’s more like 15 as a general rule, and Soarin’ takes a long time just because the theaters take so long to clear...
Lines can definitely get backed up, though, especially when rides have broken down. If a ride that’s been down comes back up, lots of people will show up and flood the FP line. Also, if other rides in the park are down, people will take their multi-experience passes elsewhere and strain those FP lines.
Even when the lines look horribly backed up, it’s usually less than 20 minutes to actually get on, though...