The iPad 2 I can see will probably still be a device talked about/used for possibly another 3+ years at least unless something extremely radical changes are brought to the table to say otherwise. A dual core processor goes a long way even if it only has 512 RAM. Sure it may not run as fast as the newest and best, but it's functional and still runs perfectly fine as it does now. The one thing that makes an Apple product an Apple product is that you'll expect your device to last and maintain its value over several years (if not longer). I would say the average lifespan of any iPad generation (at the moment) to be something like 7 years if used with care and assuming the battery can hold a charge for that long (if it's not replaced with a new battery somewhere down the line).
Example to support would be the 3GS. Released in 2008, terminated (in terms of iOS support) in late 2013 with the release of iOS 7. 5 years turning 6 in a few months, and it's still a relevant device and iOS combination. Sure you can't run iOS 7 required features, but apps had an iOS 6 version, and the AppStore allows for use of older apps for legacy devices, so you can still use the older 3GS. iPads being more durable, can still be used to surf the web, read the papers or a book, or have some fun time playing some games to relax even after being terminated from the iOS update list, like the iPad 1.
How long it should last, and how long it will last can be two separate or totally different numbers, but how long you want it to last may be the better end of the question.
