Q: How long a/c should last?

Hey, I got my car re-charge with the a/c gas. I got in the turn it on and it was frezzeing—loved it. The mechanic told me there were no leak and was ready to use. I then when on a trip about 2h 30mins there and then back. While driving I turn the a/c on and off because it got a little warm. I then got back from the trip and used it on it was okay. After paying for it to be re-gas it was gone, it only lasted me like 3-4 months. Should it really last me that long, or should it last longer? Also I've heard if you don't use it, you pretty much loose it? Now we are in winter should I use a/c with heat to help defrost it? The car I have is a Vauxhall Astra, 1.6 club. The car is 12 year old (2004). I don't know if I've been done over by the mechanic or putting it down to the car being old. Also do you think I should get re-charge again or it's a waste of money? Thanks for you help in advance. Pedro.

My car has 94000 miles.
My car has a manual transmission.