I'm expecting to get at least 2-3 years of heavy usage out of my 6th gen iPad. I use it pretty heavily. Math homework notes (Apple Pencil), 4-5 tabs on Safari, Slack for work communication, email, and reading lots of books. Including posting here.
The fact that iOS 12 is still supporting iPad Air (2013), iPad Mini 2 (2013), ... I'd definitely give it a good handful of years.
I think in 2-3 years I'll be looking at a new iPad just because I'll want more ram than 2GB, but I definitely am doing what I do with 2GB just fine right now.
My 2018 iPad is my most used device. I really like it.
My wife has the iPad 9.7 Pro. It's thinner, little faster, and reacts to the Apple Pencil faster. It was also 3x more expensive.