How long will my CO2 tank last???

We recently purchased a dual tap kegerator with a 5 lb CO2 tank. We've cashed 5 kegs and the regulator still says there is about 500 psi left in the tank (I keep the tank in the fridge, which I know impacts the psi reading- is this ok?). We haven't experienced any CO2 issues and I'm just wondering when I might expect it to run out. More specifically, how many kegs can I empty before I run out of CO2??? I use the gas to force carbonate and serve.

Thanks!!!

The number I've been hearing is 8-20 kegs. Less if you carb with gas, more if you carb naturally and only use gas to serve. I got 2 kegs emptied and about a week after hooking up the 3rd keg the 5# tank is empty. I think I have a leak. I hope.

EDIT: Oh yeah, and that high pressure gauge is junk. It'll read 500 psi until it is plum empty.

I have a dual tap kegerator with 5lb co2 and I usually get between 5 and 6 corny kegs. I carb the kegs with co2 and have a 3rd always waiting fully carbed. So I carb 7 to 9 cornys and serve 5 to 6. I also keep a full backup co2 on hand because it sucks running out.