I have a late 2012 21.5" Imac. How long will this be functional?

Define ‘functional'. It will keep working as long as the hardware keeps going. There are users on these forums who are using Macs much, much older than yours. Will it be able to keep up with new versions of macOS? No. Can it be upgraded to support the latest fancy features and such? No. Will it remain perfectly useful in most of the tasks required of it? Absolutely yes.

It truly depends upon how you define functional. It can upgrade as high as Mojave which currently is being supported. However, if Apple's past patterns are anything to go buy it will likely stop receiving updates at the end of this year. That's when you need to perhaps start being slightly concerned if you are into buying new Apple mobile devices such as iPhones because then the countdown starts to when you buy a new iPhone and discover it won't connect to your computer.

I could go on but I am one of those who has never attached my iPhones to a computer. I too used a 2012 iMac with an old OS version and do not spend my time chasing Apple and its upgrade trail. I anticipate hardware failure to be what makes me change to my next computer. In that regard we can't really say. One of my long-lived Macs ran 24/7/365 for almost 20 years before it failed. Another one, I only changed it because the I couldn't find a web browser that supported all the Internet features that were appearing at the time. I'd say that needing an up to date web browser has been the main reason for my retiring a Mac.