I had many situations where a monster dies and leaves a corpse.
When I pick it up, I immediately get the "your equipment seems to weigh less" notice, and the corpse is gone.
Sometimes it can disappear the very first turn it's generated.
I believe it rotted away but it happened way to quickly after the monster died.

There should be some minimum time for complete corpse decomposition after its generation from a slain monster, otherwise it doesn't make sense.
It happens mostly (if not exclusively) with corpses of common monsters - bats, goblins, orcs, kobolds...
Of course one could argue many monsters stink of rot anyway and the decomposition process might have already started while they were still alive but for the sake of realism, make it so that corpses "stick around" for 10-20 turns at minimum after being generated.