Laptop battery life span?

My Asus laptop has recently given me a message to replace my battery. I have the same battery that came with the laptop, and I got the laptop 2 years ago and have used it nearly on a daily basis, never taking the battery out but leaving it unplugged when fully charged. Is that a good lifespan or is something... show more My Asus laptop has recently given me a message to replace my battery. I have the same battery that came with the laptop, and I got the laptop 2 years ago and have used it nearly on a daily basis, never taking the battery out but leaving it unplugged when fully charged. Is that a good lifespan or is something wrong with the computer?

Also, what's the average cost for a replacement battery?
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