I'm still using my first MacBook Pro, the 13 inch 2012 non-retina one that was upgradeable. I had it upgraded to 8GB RAM, 250 GB SSD, and moved the 500GB spinning HD to the DVD slot. I had to get the hard drive cable replaced twice as it died on me, and these days my Mac sometimes freezes up when I move the screen.
So far it's been going for 7 years, 2+ months and counting. Ideally I would have like to have upgraded by now, but the butterfly keyboard issues have scared me off and this machine still gets the job done, so there was no absolute need to splash over $2,000 on a new one.
In the end I voted for "Whenever the machine breaks."
In a perfect world, I want to get the 16 inch MacBook Pro Apple is rumored to release later this year with their new scissors keyboard, not because I need it, I just want it. That is going to be WAY out of my price range though.
If/when Apple releases a newly designed MacBook Pro line-up without butterfly keyboards next year, I may go for one of those, at which point my MacBook Pro would have given me 8 years of solid service.