What size of aquarium are you running it on? On nano/pico tanks it should last at least 4-6 months however it depends on the tank plant load... Such as a heavily planted nano tank, running co2 at 4-5 beats per second, using a regular glass diffuser, a 20oz tank should last around 1-2 months.
It means you have a co2 leak somewhere... What kind of regulator are you using? Does it have a solenoid? What kind of diffusion method are you using? Are you using co2 resistant air line tubing?
All of these things can cause a co2 leak or co2 to escape... A good quality regulator w/ solenoid, brass check valve, co2 resistant tubing, and a good diffusion method like a rex griggs inline co2 reactor connected to a canister filter will help to prevent co2 leaks.