A. If your c02 tank is in your fridge, that gauge will be useless. The colder the tank is, the lower the pressure will be inside the tank. At fridge temps, your gauge will read about 500psi from the time it's full until the time it's empty. You'll get like 4 pints worth of warning before it drops to zero. There's nothing wrong with that, just know it's completely normal.
B. I'm assuming you have a 5 lb C02 tank.
C. I'm assuming you don't have any leaks in your fittings, tubing or connections.
D. I'm assuming you are using C02 to force carb your kegs.
You can expect about 5-8 kegs per c02 tank before you have to re-fill it. That's a linear figure, so if you have a 10lb tank, you can expect 10-16 kegs or so. I pay $1/lb for C02 so that's a bunch of beer for me.