I went Note4, Note8, Note10+. Would have been on a longer cycle between the Note8 and Note10+ had I not been a dumbass. 3-4 years seems good to me.
I'm in a plan that allows me to buy a phone at 50% off, just have to pay for the phone upfront at one time. I'm on a great plan.
Previously I'd upgrade and sell the old phone on Craigslist. I've sold 4-5 phones if I had to guess.