Server How long before my backpack despawns?
Depends what you have. The despawn timer for the backpack varies according to the highest tier of item contained.
When you die your corpse will remain for 5 minutes and then switch to a camouflaged backpack. If your best item is a torch, the backpack will despawn in 10 minutes after player death (5 min corpse + 5 min backpack). If you died with an M249 in your inventory, the backpack will despawn in 65 minutes (5 min corpse + 60 min backpack).
Be sure to thoroughly search the area where you died! The camouflaged backpack can be difficult to locate in some terrains, your corpse can be moved by jungle animals and by water (Check up and down rivers, check surrounds out in the ocean.
In the event you died with or near fire it may have been damaged enough that it no longer exists (if you died and had no revive option it is unlikely you have a backpack or corpse.)
If you die and have respawn as the only option your corpse couldn't be resolved and was removed by the server, you will not have a backpack to collect.
Detailed backpack despawn info can be found here.