It usually lasts until it leaks. AC compressors have a blow-off valve on them believe it or not. It's designed to vent if the AC system exceeds its designed working pressure. Check it when it's dark outside and see if it gets cold. IF it's just weak, but still works, then you don't have a leak and a recharge will surely fix it. If it's not working at all then you probably have a leak somewhere, and you should fix that before getting it evacuated and recharged.