If I was to park my guy in his house then leave the PC on all night (real time) would everything re-spawn?
I know your answer... "Try it"
How will I know if everything reset?

What i really want is for all dungeons and forts to fill up with prey.

Not sure, but if the amount of time your idle is more than say how long a shop keepers inventory lasts, than i'm sure is will reset that.

That's it?
No one knows?
I'll give it a try tonight.

Park and wait 8-10 hrs then hop all over the world and see who popped up.

Most cells and dungeons are set to reset after 30 game days. (In Oblivion, it was 3)
Weapons dropped by dying NPCs and ash piles are bugged and usually remain, but actual new bandits / monsters will respawn.

Did a short test.
It took 75 min of real time to go through 24 hrs.
It would take 37.5 hrs to let a month go by... not going to happen.
Can't even think of walking away from the computer for 37 hrs, getting the shakes just thinking about it.

i doubt it, when you complete a dungeon it is marked as cleared on your map so it prolly wont refill with loot and enemies

If you're on pc you should be able to manually reset the dungeons...and if not, just use the wait function.