Tony Vargas
Legend
It's driven by the rules.One of the many advantages of D&D is that it's not a video game. Things can happen even when you are not triggering a scene by entering the area. This idea that the PCs can just waltz back where they left off like nothing happened after a week off is plain and simple a failure of DMing, not the rules.
Sensibly....
If the players are assaulting a stronghold, and expect to put a pin in it, retreat, rest, and pick up where they left off, well, that's a stupid decision and no competent DM wouldn't 'punish' it.
Conversely...
If the players are exploring an ancient tomb, untouched for centuries, that only they know the location of, and they decide to retreat from a powerful construct guarding one of the inner chambers, re-seal and cover the entrance, rest up, make a new magic weapon capable of harming said construct, and come back to dispatch it later, well, that's just 'bout Smart Play, i'n'it?
But, D&D, by balancing both classes and encounters with resource attrition and prescribed day length, makes every such decision a potential point of failure. Either the DM fails if he doesn't apply time pressure, or the system fails by breaking when there's absolutely no reason for that time pressure.