So you recharge it every time it stops blowing cold. It sounds like you have a leak. The best way to approach this problem is to put UV dye in the system, run it until it's not cold anymore, then look for the leak with a UV light.

You should have a professional look at it. They have the UV dye and UV light to diagnose any leak. Every time the system leaks, some lubricant escapes with the refrigerant. If you keep topping the system off you could ruin the compressor by letting the lube run dry.

Like zeke said, if refrigerant (and lube) is leaking, you may find the leak by looking for a wet spot on the system. Unfortunately, the evaporator is a known weak point for these models, and it is horribly difficult to diagnose and replace a leaky evaporator.