How long will an endorsement last if not often used.?

They don't expire. Your CFI may want you to demonstrate proficiency in either during your BFR if you indicate you'll want to exercise that, but it's a logbook entry that doesn't have a use-by date.

As said, and endorsement is forever, and does not expire, however your currency in any aircraft expires after 90 days. You cannot carry passengers until you have made 3 takeoffs and landings. Those must be to a full stop in conventional gear craft, others touch and go will suffice.

Forever unless you lose your license completely.