How many years should a Roku stick last before changing it to the latest model?
If you mean simply in the sense of failing, most electronic products follow the “bathtub curve” of failure rate. Ie: lots of failures early, then settling into a low (but non-zero!) failure rate for some time, followed by a rising failure rate later.
Then there are the questions of: Is it the latest and greatest? Does it do what you need it to do? Do current apps support it etc. For example, my first personal computer from 1975 still (amazingly) runs, but it fails all the tests in this paragraph. I keep it for sentimental reasons and because it is cool-looking, but I am not typing this response on it! 😀
I just ordered the 4k Roku Streaming device because my local weather app doesn't run well after they updated the app on my current 1080p Roku Streaming device. The 4k one is more powerful and faster than the outgoing 1080p one.
The main issue is how long will Roku support it. Roku is still supporting some Roku 3 models that are probably 8+ years old. But, they are slow and many apps are getting bloated and slow, even on an Ultra.
If you don't need Ethernet, the Express 4K+ is a good deal, and on sale for $27 on Amazon, versus the normal $40.