How long do you run carbonfilter before changing????

running a vortex 600 and profilter 75 carbon filter. i run this setup 24/7. (this carbon filter is the kind that you can flip over and use the other half of canister and carbon on that half) how many months should i run it before flipping it over and using the other half? 5 months ? 6 months? 8 months? the guy at the store said the carbon filtration should last between 12-18 months depending on how much you run it... anyone? THANKS another question is how much does the carbon refill run for when i need to refill it? THANKS