All depends on your machine, some can go a few thousand feet before they start to run so bad that you need to re-jet, others need it more often. It depends on your machine and how it runs but if its a 4 stroke, probably should will never have to change jets unless you ride across a really big elevation change.
going up in elevation you will jet down, going down in elevation you need to jet up. Also you will jet down for warmer temps and up for colder temps. Only go (up or down) one jet size at a time.