You get 1 "chunk" of rest xp per 8 hours in an inn(5% of a level). You can get a max of 150% of you level worth of rest which takes 10 days real life time.

1 full level of rest takes 160 hours to accumulate, or 6 days 18 hours.

If you are full rested it usually takes a few hours to go through it all unless you are boosting, then it could go in a few minutes. If you always log out at an inn in the upper level 70s you have to play pretty hardcore just to catch up to your rest.