Choose a subscription based on your consumption. Each month or two, depending on your subscription, swap your canisters for new ones.
The subscription is designed so that you never finish all the gas in all the canisters, so each month you will be giving back a canister or two that still has gas left in it. This has been factored in, and means you never run out!
If you are running out before our swap, this is an indication that you should probably move up a tier.