Originally posted by Weaver:

Anything outside of player base ( https://valheim.fandom.com/wiki/Player_base ) range despawns within a couple game days.
Anything dropped into 2m+ deep water despawns much quicker.

As a side note to this, I've tried to dig wells in my bases, so I could just throw stuff in them, but I've never been able to get that 2m+ depth by pick axing.