So not sure how long it took because hadn't checked since clearing out the mole den right next to red rocket settlement after in-game day 1, but currently on in-game day 29 and went back there

As expected, the moles inside this interior cell location had respawned - also as expected the ground loot in plain sight had also respawned.

But unexpected and cool, was that the locked safe had respawned - as locked again. So not only did I get loot, but the xp to re-pick the lock.

Have not checked exterior cell locations like Concorde - maybe those exterior locations with wall mounted safes won't have respawned with locks,but the interior cells seem to do so after whatever X in-game days.