If you have to add air more than twice a year, it's time for a new tank.
Just set the air pressure in the tank 2psi lower than the cut on pressure of the pressure switch.
If the switch is a 30/50, the tank should be 28psi.
From the sound of your post, check the air pressure, add if you need to, and then check it again in 3 or 4 days.
If the pressure has fallen any from the amount you put in it, it's time for a new tank.