I read reams of white printed paper (eg, the paper that comes out of your laser printer) I don't get tired. This is not back lit LCD we are talking about.
In any event, eink is not going to jump from the current gray to blinding white. It's like somebody sitting around saying i eouldn't want to have a billion dollars; it's too much money and i eouldn't know what to do with it. Well you're not getting a billion, so don't worry about it