Probably 5 years is about right as all systems lose gas whatever. If you run the air con all the time you'll lose less gas.
As for £90 :? I assume that's a stealer. I always have mine done at my local BOSCH centre and that's £50/ The machines they, and probably BMW use have the BOSCH name on them and do the whole job with the push of a button. 45 minutes to an hour and your done.