I'm thinking of playing a necromancer next and, well, collecting dead bodies. If I make someone a dead thrall and then kill them, how long does the body last? The usual time? Forever? What if I leave them in a house or another cell that doesn't reset?

There are very few bodies that are permanent, and only a tiny fraction of those are thrall-worthy. The reset time for non-permanent bodies is 1 day. You won't be able to collect them for very long.

You can only use dead thrall on humans, so no werewolf companion for me when i made a necromancer.

And on occasion the body will reset alive. Imagine my surprise when I walked into my breezehome and found a hostile bandit chef.

you didn't use the ritual stone ?

I have an issue where former thralls that died will show up out of the blue. It's happened at least 10 times now. Often, they aren't even hostile. If you cast a spell next to them they will attack.

Sounds like atromancy may be the way to go then. I really don't want to babysit my thralls.

it's annoying to babysit them but it's worth it IMO it's one of the strongest spells in the game if you know which thralls are the best the only thing Is to know which thralls to use