When the tank is empty of water the air pressure should be 2 psi less than the start pressure of the switch.
It is very likely that your tank has failed; either the bladder, or the tank has a hole rusted through it. Nevertheless, here is a way to test it.
First check the pressure gauge and tire gauge you are going to use by measuring the air pressure with the tire gauge and looking at the water gauge. THEY SHOULD BE THE SAME.
If the gauges are not the same, here is a way to put proper air in the tank. After the pump runs and shuts off, shut the pump power off and SLOWLY release water from the tank until the pressure switch clicks to start the pump. Then measure the air pressure with the tire gauge.
Now open the water drain and add air to the tank until the pressure is 2 psi less, as measured by the same tire pressure gauge, than you measured in the previous step.
Shut off the compressor (or however you added air to the tank) and wait 30 minutes. Measure the air pressure again and if it has lost a lot of pressure then the tank or bladder has failed.
You should get a decent pumping time after you recharge the tank. If the pumping time is shorter after a week or so then you have a failed tank.