LEDs are typically quoted as having a lifetime of around 25000-50000h. It's not as simple as it may seem, though, because LEDs very rarely die, they simply fade. The quoted lifetime is generally the point at which the light has an output of 70% of its original value; 70% is a pretty arbitrary value which is deemed to be the point at which you'll notice that it's not quite as bright as it once was.
So if an LED has a lifespan of 25000h, and you use your reader for 2h a day, that means you may notice a lessening of the brightness after 25000/2 = 12500 days, or about 34 years.
ie it's not likely to be a problem with an eInk reader
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