Not much to say here ...
I think Harry and Billi answered your question.
But basically, as long as the ereader is in sleep mode and wifi or 3G (if available on your device) was turned off, there is zero or very low battery consumption. The battery is used mainly when turning or refreshing pages. That's why they are so good on that. It is different on LCD devices, which regardless of the page you are, keep it on will and is consuming battery all the time, until you turn the device off or goes to sleep.
The life of an eink device is more tied up to the protection of the screen. The device itself can last several years without any issue if you don't break it.
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