Also, in a perfect world and in theory an AC system in a car should never leak in the first place.

If your car is low on refrigerant then it is leaking somewhere, it doesn't get used up like gasoline, it's just taking advantage of chemical endothermic and exothermic reactions when they change states of matter (Hence why it's called an AC condenser and evaporator)

The AC stop leak is inside most recharge kits because merely adding more R-134a won't solve the problem if it leaks out again anyways. The only time you charge an AC system is when you replace the compressor or AC related parts and have the old R-134a (or R-12 in older cars) taken out with a vacuum pump and stored to be disposed of. All that stop leak does is expand the rubber seals so that any cracks they have due to moisture swell up so they don't leak as much but it's not permanent. It's just replacing all the rubber seals/ o rings and then draining and refilling the AC compressor is a long job that can be stupid expensive.

So use at your own discretion when "refilling" a cars AC system, and make sure you know what you're doing with it cause releasing it in the atmosphere if your compressor blows due to an overfill can land you a hefty thousand dollar fine depending on your state.