How long your camp stove fuel lasts depends on how much you use it and how fuel-efficient your stove is. Consult your stove specs to see if the brand listed the burn time for a single fuel canister. Typically, the rate is 60 minutes of burn time per 230g canister, but air temperature, wind, and elevation can all affect that.
If you’re worried you won’t have enough fuel for a backpacking trip, pack an extra canister. You can get small ones for backups. It’s better to have more fuel than you need than to run out on the trail.