I've got a 15kg Butane tank, up to about 3 years now and I have barely used any of it.
You can usually swap butane/propane for gas BBQs, you just need to swap out some connections. BBQ I bought came with propane fittings and took 10min to swap for my Butane tank
And pro-tip. you can check how full they are by 1. Checking the weight, 2. Giving it a shake and 3. Feeling the side of it, as the liquid gas level drops you feel a cold area and a slightly warmer area. Where they meet is the fluid level. Another option to make it easier is pour some warm water down the side of the tank, the gas area will stay warmer longer and the liquid area will cool much much quicker.