There's dungeons I've cleared at the beginning of the game that still hasn't reset with new enemies 200 hours later (real time).

Which Dungeons?

I beleive it's 30 in-game days? At default timescale of 20 it would equal 36 hours (30*24/20).

Areas that are marked cleared on your map respawn after 30 days (720 hours). But be aware that each time you re-enter a cell you reset the timer. If you go back into that cell before 30 days are up you will reset the timer and it will be another 30 days before the cell respawns.

As a side note, if you do not clear a dungeon, it resets in 10 days. Not sure if that applies to Loot and or Ore Veins as those are normally 30 day timers also.

If I go to a cell with a dungeon it resets the time even if I don't enter the dungeon?

A dungeon cell begins when you enter the dungeon. And if you go through a second or third door in that dungeon (with loading screens) you are entering more new cells in that dungeon. Many dungeons are composed of multiple cells.

If you stand outside the dungeon you are not yet in the dungeon's cell.

My game must be bugged then. There's dungeons i've cleared over 100 hours ago that still are void of enemies. Dungeons I never went back to so the respawn timer couldn't have reset.

I don't even think you have to enter it, isn't it enough to just approach the dungeon close enough to see the map pop for the dungeon?

I have kept some places empty for a long time just by running by them all the time as I go to and fro. Note, I have not checked inside these places, I just don't see them on the battlement walls for instance.

Battlement walls?