A 5 lb tank will register the same on the gauges whether or not there is 5lbs or 1lb. The pressure doesn't start dropping until the last of the liquid evaporates, and there is only gas in the tank. If your tank is not getting filled all the way, it could only last a couple of weeks, but still read 800-1000 psi when you bring it home from refill. I have had that happen before. Weighing it before and after is the only way to be sure you are getting it filled. It also depends on the ambient temperature. Higher temperature, higher pressure.