My air conditioner works for 20 minutes but then blows out hot air.

This could be any number of things, but you will need to verify that the freon re-charge was done properly, you don't have a slow leak somewhere, the low and high pressure switches are good, and that the condensor fan is actually coming on when you turn on the A/C.

Check that the radiator fans both work with the A/C on. If the fans do not work the system will over pressurize and shut down the compressor until pressure drops low enough. If it works longer at freeway speeds than at slower speeds the fans are the problem. The air gap on the compressor clutch can not be to large or it will have trouble engaging when it is hot (works fine when cold but can't re-engage after clutch cycles).