Will items eventually despawn if I place them in a container or drop them on the ground?

In my experience no, I've dropped lots of things and left many things on the ground and they're all still there. However, there's most likely a limit eventually. It could be in the 10,000s, meaning you'd have to drop 10,000s of other items before it removes the oldest one, but it will almost certainly work that way.

If you're worried about leaving something for a while and then coming back to get it, that shouldn't be a problem. It's just that unless the save file can grow to an infinite size, there's going to be a limit, even if it's so high it doesn't realistically matter.

I really wish we could get some confirmation on that. I found a chest that I can actually place in game and use as a stash box, but I'd hate to load it up with stuff and have it vanish 80 hours in or something.

I think Larian initially promised that we can use the homestead as a place to store our treasures. There is for example a chest in the side room of the Hall of Heroes. I would guess that stuff stored there shouldn't disappear. But of course I'm not Larian, so that's no final confirmation.

Yeah, I know what you mean. In games like BG, you could do as you say and just use it as a stash box. It always felt scary because you didn't quite trust the game to work as described, but it did. In the original Diablo, gold was so worthless that people would use it to draw pictures on the ground of Tristram, and it would always remember that picture, made up as it was from dozens or hundreds of gold piles.

But as I say, there's very likely to be a hard limit to remembered "out of place" items. An official confirmation would be cool, although it will most likely be so high it doesn't matter. I guess for now, just don't leave anything there that would depress the hell out of you if it vanished (in Fallout 2, quite apart from any item limit, items left in your car would randomly get deleted; now that was annoying as hell).

Items you put in the world do not disappear, unless an NPC walks by and picks it up. E.g. animals eat food and some NPCs love potions.

Cool, thanks for the information.

You can safely stash stuff in containers. Some NPC AI can pick up items on the ground, though (opponents can pick up health potions and drink them, for example, but I don't know if that applies to any friendly NPCs).