At least two years, probably 3 to 4. That's the current iPad Air. Add a year for the next one.
Statistically, the batteries should still have 80% of their capacity after two years. That's if you do a full recharge (0 t0 100%) a day. Most people don't push their iPads that hard.
But that is a statistic. Any iPad may fair better or worse.
As far as the software goes, the iPad 1 was introduced in 2010, and is pretty much done for. People still using it are like people using older computers. The old software does what it did, and new software is hard to come by.
The iPad 2 introduced in 2011 has probably seen it's last update with iOS 8; which really is pushing the hardware to the limit.
So, if I had to make a guess, the current iPad Air is probably good for three to four years, both for batteries and software updates. After that it will still be usable in a more limited way. Don't expect it to get every new feature, if for not other reason than it may not have the hardware to support those features; like Touch ID which is 'probably' coming out with the next generation.