Is there a certain amount of time that mushers have to stay at a checkpoint?

yes. example: every musher in the Iditarod race can spend 24 hours only once at a checkpoint. so it would be a good idea when they are tired to stop at a checkpoint for 24 hours since they only get to do it once. they all have limited time at checkpoint. if u r confused, go 2 the iditarod website.