Warning: conjecture ahead!
It's impossible to predict with certainty - but it helps to think about what is politically expedient for Apple, what is technically feasible (due to device specs), and by looking at past trends.
Importantly: Apple has no stated-policy, but iOS updates seem to always be available for at least 2 years after a product was discontinued (excepting the original iPad)
Product Launched Discontinued Last iOS Update Date RAM
iPad 1 2010-04 2011-03 (1yr) 2012-05 (iOS 5.1.1) 256
iPad 2 2011-03 2014-03 (3yrs) Current 512
iPad 3 2012-03 2012-10 (7mo) Current 1024
iPad 4 2012-11 2014-10 (2yrs) Current 1024
iPad A1 2013-11 Current Current 1024
iPad A2 2014-10 Current Current 2048
iPad 2 was sold from 2011 until March 2014 - a long time - cynically, because it represented an 'entry-level' value-proposition from Apple with what I believe was a good profit margin compared to the relatively far more expensive retina iPads - so it wasn't much of a surprise to see the iPad 2 out-last the iPad 3 in 2012 and almost the iPad 4 (which was sold until October 2014).
I think iOS 9 will be the last iOS update for the iPad 2 - look at that 512MB RAM figure, that was probably the reason the iPad 1 with its tiny 256MB RAM wasn't given the iOS 6 update. 1024MB RAM gives more breathing room - while the iPad 3 is older than the iPad 4, they are very similar internally (the major change in the iPad 4 was the Lightning connector) - so I expect them both to stop receiving iOS updates on the same date.
I think the iPad Air 2 is the new iPad 2 and will serve as Apple's new 'entry level' tablet so it will have a longer market lifespan and will probably be discontinued after it's at least 3 years old and receive updates for at least 2 years after then (though I'm surprised the iPad Air 1 is still in production given the iPad Pro 10" is now available).
Model Discontinued Last iOS Update
iPad 2 2014-03 iOS 9 (2016)
iPad 3 2012-10 iOS 10 (2017)
iPad 4 2014-10 iOS 10 (2017)
iPad A1 2016-10 iOS 11 (2018)
iPad A2 2017-10 iOS 12 (2019)