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.

Detailed backpack despawn info is here.