How long do I have until I have to replace computer battery?

Right now I just got a notication saying "Consider replacing your battery" and it has a red X on the battery icon. How much longer do I have until I have to replace it? I'm using a Lenovo thinkpad X200 and I have it on power saver mode. I checked the batteries online and they go for $40 but I'm broke so this is why I wanted to know how much longer I got.

Added (1). Lenovo laptop thinkpad X200

Added (2). Everytime I use my laptop, I always keep it charging even if its at 100%

If you are running it off the ac power supply you don't ever have to replace the battery.