When I was using Android phones (2008-2014), I used to upgrade every 6 months or so. I was on T-Mobile at the time and the loyalty department was quick to give out full upgrade pricing. In 2014, I switched to the iPhone 6 and, since then, have been keeping my phones a lot longer. My iPhone timeline is 6, 6s Plus, X, 11 Pro. I was actually considering keeping the X for another year, but my wife's 11 Pro gave me serious phone envy and the $400 trade-in value for the X (not a bad deal for a 2 year old phone) made the decision to upgrade easier.
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I'm still using a mid-2012 non-Retina MBP, mainly because my primary computer is a desktop PC. With a couple of upgrades (bumped the RAM up to 16GB and installed a 512GB SSD), the MBP is still chugging along fine. As much as I'd love to have one of the new MBPs, I can't justify spending over $2,000 on something that I'd use casually.