How long does it take or what do you have to do to get a cleared settlement to respawn with enemies and items?

It seems to vary. Check every couple in game days to see if the [cleared] marker is gone.

Read 20 days somewhere, however if an radiant quests want to use the location it might respawn faster.

Had an fun bug where an cleared location was reused in an radiant quest but only the boss at the end spawned.

made the quest pretty much an walk in the park

Area's marked "Cleared" on the map will respawn in 20 days / 480 hours; in-game of course.

All other areas should respawn in 3 days / 72 hours.