It's one of those questions with many answers.
Normally, the lower your mash temp, the longer you want to mash, to give everything time to convert.
If your grain bill is all or mostly grains with good diastatic power like 2-row or vienna, 60 minutes is normally fine.
If you're brewing a light lager or something with adjuncts like corn or rice that have no diastatic abilities of their own, a 90 min mash is preferred to give your diastatic grains time to convert the adjuncts.
I think (but may well be wrong) that going much over 90 minutes doesn't really do a whole lot - conversion wise.