can't say I've tried to extend any, but if you applied a 2012-earned GPU in June 2013 for a Dec 2013 flight and it never cleared, it would be pretty bogus if they only gave you 1 month to use it. not that UA cares about fair, but it seems the appropriate thing to do would be to put a 365 day expiration on them but suspend accrual while upgrades are waitlisted.