How often does a properly functioning a/c system need to be recharged?

Never. You should only need to recharge it when it has a leak or after servicing the system.

The dye is supposed to leak out to highlight where the leak is, it obviously won't stay there forever so you'll want to check the system for the dye within a week or two after putting it in and driving it with the AC on to let it circulate.

Many mechanics recommend every 4-5 years. Dye lasts pretty much forever, both inside the system and outside where it's leaked. Unless you blast the bottom of your engine bay with very aggressive cleaners, that green shit will stay.

>>17768830
>recharged
never
>checked
once every year or so

but all hangs on the
>properly functioning a/c system

Since OP is asking about AC let me ask this, my AC works fine in town and blows a good amount of cold air, but when driving on the highway you can barely feel it move air even on the highest setting of the fan.
What could be causing it?