The Air Canister is empty when crafted. It can be filled by right clicking on an Air Generator with the Air Canister in hand. It will slowly be emptied if held in a players inventory while they wear a warp helmet in space.
While it is emptied a bar at the bottom of the item will decay just like armor or weapon damage. When the canister empties completely it will return to being empty and can be refilled an unlimited number of times.