The battery management used in the Kindles is quite sophisticated.
You can expect at least a two year life.
But of course not as sophisticated as that used in your hybrid car, where batteries start at $10,000 (and up).
With those, you can expect at least a ten year life.
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And I bet there are not a lot of people who are setting aside a $1,000/year of their savings in fuel, to replace the battery a decade from now.