How long after leaving a game will a Storage Crate despawn?

So items on the dock will for sure stay there longer than items in water as those will sink and despawn. Outposts reload differently than other islands I believe though since they do not have to worry about respawning enemies, animals, etc. and only the items in barrels. the thing that works in your favor here for doing something nice for other people is that when you leave a server someone else somewhere is also joining a server. They try to consolidate servers so it is likely that people will load into the spot you were just at. Loot items that were touched also stay longer on any island compared to items that spawned there and were not touched. I apologize for not being able to answer with a simple time frame but the general answer is this does help people and I can almost for sure guarantee that someone will use those supplies you leave.