I went 3-4 years, roughly, between iPads for a while (original, iPad 4, 2016 pro, now 2020 pro). Before, every time I updated they had some pretty impressive improvements. But 3.5 years into me 2020 pro, there’s absolutely nothing in the latest models that makes me the least bit interested in updating anytime soon. I mean, it’s plenty fast, what is the M2 going to do? Or M3 in the next one, inevitably. There are a handful of apps that can’t run at full capacity, I guess, but I don’t really care about rendering 4K footage in Resolve on an iPad since it’s much easier working with files on my MacBook anyway.
TL;DR - I foresee my current iPad lasting another 4-6 years.