2005 Ford Taurus AC in taurus

I have not had AC for two years. I have had two mechanics look at it they always fix a cap or hose here or there and it will work for a couple of days. Then all the freon leaks out. That second mechanic has fixed it like three times and I have spent alot of money and still have no air. Last time they put some kind of stop leak and it worked for two days. Any idea what is the problem with the AC. I see so many tauruses driving around without air.