After killing an NCP their bodies seem to stay less than a day before disappearing, leaving only their weapons laying on the ground. Both in the open world and in dungeons.

Are you using 1.2? One of the bugs fixed in the most recent update:

Oh man if that's the case I hope there's a friggin setting we can set, say, 100000days... having dead bodies laying all over the world is important to some. Have things just disappear (without even properly rotting 1st...) reminds me too much of pathetic limited resource no-depth no-brainer games that try and mimmick deeper games but in reality are just shells and have no depth at all, or persistence.

(Troll humor)
You sure they were actually dead?
Did the dingos take them?
Wild rabbits are carnivores in this game...
Did you want to burry them yourself?
You killed them... yet you are worried about where they are now?
Were they empty?
Necromancy is popular here... Just saying...
Starting a dead body collection?
The rule is... "You break it, you own it.", not "You kill it, you own it."

I have a different problem, thieves don't disappear at all. I have five thieves in Riften that still are staying dead for the entire game.

Yes, I am using 1.2. Actually, it only started happening after the patch.

You want them to stay dead... they have a hard time breaking-into houses that way...

If they didn't stay dead... they would be zombie thieves!

(I get what you are saying. They STAY, and they are also dead. Just having fun with that cute reply.)

Those may not clear, because you visit that city a lot, or never left that city long-enough for them to be removed. The game will leave many things around, just in-case you need to take the things inside them.

I don't know how citizens and guards can stand living with death laying all over town... You would think that towns would kindly move the dead to a cemetery or cover them up until the CSI units arrive!

Yes, because your PC will just love you for an eternity for that eh?

But then again, it's good to see a dead bandit I threw off a mountain at the start of a dungeon and to come back later and see it's still there.

I never had a humanoid NPC corpse disappear quickly before or after the latest patch. However, I had a dragon corpse vanish probably an hour or two of in-game time after I killed it. I was headed to the giant camp north of the West Guard Tower outside Whiterun. Got a dragon on me at the guard tower (ancient one even), killed it, then went to pay some giants a visit (was attempting to train my destruction a little... such a PITA to level that skill). Spent a few minutes on that, and returned to the tower where I left my horse... and the skeleton was gone. Only had that happen with a dragon though.

This happens to me as well. A dragon slaughtered 4 guards outside Whiterun. I took what loot I could hold and fast traveled to my house inside Whiterun. I then went back out to pick up the remaining loot but the bodies were gone.

It seems to be affected more by the number of loading screens than ingame hours or days.

Or a couple stuffed into a fire-pit with a batch of food baking for any canibals that may pass by, or 5 stacked up like cord-wood on the side of the road as a warning, or... etc. Can really add to the atmosphere depending on how you play, especially if you RP.

It's important for a game engine to clean up dead bodies because it costs GPU power to render them. Without this cleanup, your game will eventually slow to a crawl.

Some games do cleanup in a fun way, like Giants: Citizen Kabuto. After the NPC died, small scavengers would eat the body. While they were eating, the body would slowly clip into the ground, at which point the game would remove it from the world.

I realize that bodies should clean up but i at least one them to stay for the 10 days that they are supposed to.

1 day is more than enough. They need to go away after about 5-10 minutes at most. Beth could add an arcade style flashing body like in old beat em ups.

An entire game day is too long for crap to linger. I have issue with stuff being there after 20 hours of game let alone 30 minutes.

Regardless of the way you feel, I was under the impression that bodies are supposed to stay for up to 10 days before they disappear.