Can't give you a time scale but maybe I can explain a little.
From the moment that anyone first uses a battery and recharges it, its capacity will be reducing. Initially it is almost unnoticeable but, as it grows it becomes more obvious.
In time you will need to replace the battery. When that will be depends very much on how you use it and how often you recharge it. At a wild guess, in probably a couple of years time.