We have a 2008 Focus with a poor a/c system. It has been serviced/re-gassed over the last two years by our local Ford dealer, who have found no leaks during investigation. It works ok initially for a while then gradually gets worse and worse over a few weeks until it eventually ends up just blowing out slightly cool/almost lukewarm air.

It is left on all year round but the car is mainly used for short journeys, is this likely to affect its performance and necessitate the constant re-gasses?