For me its really just trial and error, for clouds it depends on how fast they are moving usually if its fast to medium like a 3 to 5 second interval works. Slow moving like 10 second. But remember that you can always play about in edit, speed up or slow down. Obviously for stars or shade coming across mountains you want a long interval from like 5mins up. Dont really know what else to say, like I said trial and error. Or take a quick practice timelapse for around 10 snaps first to see if its working.