text: do you need a scuba lisence to buy spare air? those spare air canisters that are used for scuba can you buy those without a scuba liscence? for snorkeling in fresh water? how long do they last? good question actually. a reputable dive shop shouldn't be selling them to a non certified diver. they have a reg similar to that used on a set of normal regs, which require some training on, to be familiar with it's use and limitations. you also need to be familiar with how a compressed gas works on the body and some of the physics involved.  there are a couple of sizes of spare air, so gas volume will be part of how long a can will last as well as the depth it's used at.  mine is the 3 cubic foot (largest i think) and allegedly holds 50 typical breaths when used at the surface. this number goes down substantially when you  hit 2 atm and so on. about 25% per atm approximately. i don't think they are what you want, for what you intend on doing. they are refillable, but i don't think the cost is worth it for you overall. mine's sole purpose is to get me out of a tight spot, should the need arise. fortunately (knock on wood) i haven't had to take it out of the holster.  you could get certified for just a little more than the cost of one of these. just a thought.