Running out of gas should be a function of volume, not time - ie, how many kegs you cycle through the tank, not how long they are hooked up for.
You may only get 3 or 4 kegs out of a 5# tank if you are force carbing. Think about it - you are dissolving 2.5vols of CO2 per keg, thats 10 cornys of CO2 out of your tank, just to carb it up. Then you need to factor in the gas needed to push the beer out - which adds up to quite a lot; a full corny at 12psi and 37 degrees is a pretty substantial volume of CO2 at room temp.
If you want to stretch your gas out a little, try naturally carbing the keg. You can always adjust your level of carbonation once you hook it up to the gas.
If you do actually have a leak, make up a solution of water and dish soap and brush it on all your connections. It'll bubble if it's leaking. One trouble spot you may want to check is the nylon washer that seals the connection between your regulator and keg. Personally, I don't like those, and would recommend the permanent brass w/ o-ring dohicky that screws into the tank.