Sorry for replying to an old thread...
Unfortunately there is no single answer to this question, because it depends on the chemistry, quality and use of the battery.
If you let the battery deplete below a certain level, the chemistry will start to break down.
If you charge it too often, the chemistry will also break down.
And if it's a crappy battery, the chemistry will break down regardless of what you do.
Generally, quality batteries made in the last few years will last at least three years. After that you will start to notice a decrease in battery life. After that it depends how much loss of battery life you can tolerate. Once you you get to the point that the battery life is half of what it was originally, or the battery never reaches the point where the charger shuts off, it's probably time to replace the battery.
It may also be good to inspect the batteries visually every now and then. Leaky batteries should be replaced immediately.
5 years would be a lot for a battery that gets used every day and recharged often, but for a lightsaber that gets used and re-charged once in a while, I would expect a lot longer life.