Hard to say exactly. Really depends on how much you step into it. I usually refill it when it's half emty. TR overflows are one gallon. In the almost four years I've had my kit on, I've never come close to running out.
How long..well its like a drug addict with a crack rock in his pipe..how long..if your constantly putting fire to it..it will go pretty fast. I've been getting about three weeks per fill.. I drive moderate Thats 1 dollar per week for my habit