Well for what it is, it can last for a very very long time, Apple will replace your Battery so it can last for as much as you need the device. For browsing the web, reading eBooks, you're pretty much sorted.
However i'd expect a product cycle like the iPhone and alot of the other products Apple has, updates every year. Next Year we might see a faster processor, The Camera, Possibly 4G connectively depending on how fast networks expand, but what other things can you add? Not alot.
For alot of people, it will last for years and years - but Apple will continue to release faster devices, with some addons and make certain apps obsolete thus 'forcing' you to upgrade.