Even if your system is still working for the most part, it may still be time to consider putting in a new AC kit. Because let's face it, those factory cost-cutting measures likely mean the car air conditioning system that came stock in your car doesn't actually keep you as cool as you'd like to be. Not just on hot days, but the days you need it most. Stock AC kits don't typically lower temperatures as well as the top performing systems, so most of us could do well to upgrade, in the interest of maintaining a comfortable driving environment. And just because it's not vital to engine operation doesn't mean it doesn't have impact on how well your car runs. A low functioning air conditioning system will reduce your effective miles per gallon, costing you more money over time than a system that can handle your climate control needs more efficiently.
In a Car air conditioning system the refridgerant gas needs to be recharged completely within four years from the manufacture date and then again at least every three years after that - preferably every two for maximum efficiency.