Im an a/c engineer by trade, the aerosole cans work but arent very good, the a/c system is a criticaly Charged system and requires an exact amount of refrigerant to work properly.

Most places have offers on ac recharge, although i dont agree with the way some garages do it i would reccommend it over an aerosol, iv done my own so i knew it was recovered, evacuated, purged and recharged properly.

Once its working use it reguraly to keep it working, even in the winter run it for 15 mins a week, this keeps the compressor shaft seal lubricated to prevent it leaking and loosing its gas.
 
The aerosoles usually work, but for £49 itl be done properly and if used reguraly should last the life of the car......unless a stone punctures your condenser
 
It should be a sealed system so shouldnt loose its charge. Normal culprit is a small weep on the compressor shaft seal, regular use keeps it lubricated and sealed. If its not that theres a small leak elsewhere, you could either get it repaired or get it recharged every 2to3 years.
 
are these the aerosoles that you just set off and leave in your car?
 
The one that kill the bacteria go in the footwells.

It's a a sealed system, the air con, needs recharging by the valves under the bonnet.