For how many years can I expect to use an used iPad 1 if I buy one today (as of 2014)?
Everyone is looking at this from the (entirely reasonable) POV of someone that wants to keep up with the latest IOS updates. Understandable as they often fix bugs and enhance the O/S greatly.

But from a strictly utilitarian point of view, the answer is, "as long as the hardware works."

I have an old SE/30 sitting in my office. I haven't turned it on in a long time but last time I did, it worked fine. So more than 20 years later that SE/30 works just fine.

"For how many years can I expect to use an iPad 1" depends COMPLETELY on how you plan on using it and what your needs will be 2 to 10 or 20 years down the road.

If you can download and use all the apps you need NOW without ever upgrading them or the O/S then you can use it till the hardware gives out. If all you need is a simple web browser, mail program, etc. The same applies.

If, however, you need to keep up with technology. If you need to have the latest and greatest apps and operating system if you need to be compatible with all the NEW stuff out there, perhaps an original iPad would not be a great investment. Save up your dollars and buy a newer one.

The wife and I bought an iPad 2 in May of last year for a specific purpose: POS for our small business (square) and streaming music plus some casual web browsing. there is no reason why it won't still be able to do that 2, 5 or even more years in the future as it doesn't NEED to have the latest and greatest to do the job it is currently doing.

If that is the case with you, buy the old iPad and be satisfied with what you have.