I was on a 6-year plan which was good... pay it off, no payments for a bit, then, get something new.
I bought my 2010 after my 2004 was paid off. It was a little painful to get back into a payment... But, I have made a series for screwups since then... the 2010 was the only 2WD truck I ever had... got stuck in a bad situation and vowed to never do it again... so, I bought a 2013 to fix that mistake.
Then, the family came along and I started looking at SUVs. I looked at the expedition and for some reason found myself in a 2016 F150. Theyre roomier in the back than the 2013 ram and I convinced myself that a camper top would solve my space issues. So, bought the ford... honestly, didn't like it after a year or so, but, was going to live with it... Then, my wife and I decided to buy a camper. The F150 was maxed out and we were already thinking about a larger camper over the next couple of years... SO, I bought the 19 Ram 3500. I wanted out of ford and back into a ram. also needed something more safe for a family growing even more as we have twins due any day now.
Having said all of that, I intend for this to be an at least 6 year truck again. There are 2 ways to look at the economics and depending on how you like to live your life. When I bought my 2013, I put no money down and only increased my payment by like $10/mo. To me, that was a no brainer. I still had payments, but, got a truck that solved a problem for very little extra money.
After that point, my economics got bad though and I have been increasing payments every since. To me, the best value is around 3 years, 60,000 miles short of waiting until you're paid off and have full equity. I could be wrong, but, I have always felt like it goes from being a "newer truck" to "just another used truck of many" for potential buyers after those thresholds and the value drops faster. If economics is your only driver, drive it until the wheels fall off or youre spending more in maintenance per year than a new one would cost.