I think there is a mximum of about 10-15 seconds, if no player ship is within tractor range of the floating loot, that you can return to the scene & recover it. You can test this during missions: one often winds up chasing NPC's a bit during a mission, so the resulting loot gets spread out. Time how long you can move out of range, and return to it. Another way is to select a loot item with your mouse & fly out of tractor range, and time how long it stays on your scanner...