Like a number of MR benefits, the late checkout is left intentionally vague, and is up to the property. Just like with the upgrade benefit, a property can largely do what it likes. In either case, you (a plat, gold, or silver) can be told "Sorry we don't have a (late checkout) (upgrade) etc. available." Not much you can do about it. Both benefits are upon availability. The property determines when they have availability.