If I leave something/drop it on the ground, how long will it stay there until it unspawns?

As far as I can tell from my testing, items will stay on the ground indefinitely as long as you're close by. They only despawn when you're a significant distance away.

As a test, I dropped an item on the ground in a safe area (a stable), then left the game running while I made myself some lunch. When I came back about ten minutes later, the item was still right next to me. This is evidence that there is no time limit for items despawning.

Another time I killed a Hinox which dropped some strong royal weapons. At the time, I could not carry any more weapons, so I walked a few hundred feet to kill a nearby Lynel. After killing the Lynel, I went back to where the royal weapons had been, but they were nowhere to be found. This shows that distance affects when items despawn. However I have not done any tests for how far you need to go before that happens.

Though the breadcrumb idea is clever for navigation, I suspect the mazes are a little too big and that items will begin to despawn as you traverse the narrow corridors, which may get you even more lost than you started.

I found that there's an item limit also, as a joke, I dropped a bunch of rare stuff to "leave it there" (I was with friends) but when I dropped my 4 lynel guts, 5 giant ancient cores, and 7 ancient cores, the lynel guts all despawned and 2 of the giant ancient cores did too (I immediately picked them up)

Note: I did this in the southern oasis in the Gerudo desert so that might have contributed because there were a few palm fruits in the trees