Yes, there is a rule.....BPM determines the length of a bar and thus the length of your loop.
As Bandcoach stated 1 bar at 80 BPM = 3.0 seconds......however as we are talking about fitting one piece of music into another it's quite possible to incorrectly shoehorn a 78 BPM sample into the same 3.0 seconds or cut your 1 bar 80 BPM loop too long or too short, which is exactly what people used to do when editing hardware samplers which displayed time as gibberish numbers (in samples) rather than seconds, thus resulting in people editing loop points without any point of reference other than the very loop points they were playing with, not a very good idea in my opinion, all this resulted poorly cut one shot samples which needed to be triggered in repetition so that they sounded like proper autonomous loops.....that's right there are basically two kinds of loops, the shit which requires constant triggering to compensate for sloppy editing, and properly cut loops which can be triggered as a one shot or run autonomously for as long as a note is held.
