How long does an iPad last?

Just a thought that I had earlier when reading the paper. A neighboring high school will be distributing iPads to their students for a $75 yearly rental fee. Sounds pretty nifty, and I'm sure it'll be a great boon to the students and staff there, especially as textbooks make the transition to a digital medium.

One thing did stick out to me though. The IT lady at the school said that an iPad typically lasts six years, and that'll be their upgrade cycle for it. Is that really true? Seems kinda hard to know for certain, especially considering the original iPad is a bit over two years old, scarcely a third of the quoted time, and it is starting to noticeably lag behind its descendants (Although, to be fair, they had custom apps written for their devices, so I would assume that if they never update the software and the apps don't change much, as long as they run fine today, they will run fine down the road.).

TL;DR: How long do you expect your iPad(or other such device, really) to last you? Do you upgrade every year? Every other year? Still rocking that original iPad?