In the days before iPads, I was an every-three-years kind of person with updates to my desktop and laptop (tried to alternate so I wasn't buying two machines in the same year).
My most recent laptops were a 2008 unibody aluminum MacBook which was replaced with a 2011 11" MacBook Air.
However, with the iPad Air, iPad Air 2 and now the 12.9" iPad Pro, I've been using the same 2011 MBA ever since. I really don't require the laptop very much at all, so I haven't felt compelled to replace it. Other than the battery which will barely run the machine for 2 hrs, it did everything I needed a laptop for.
Just a couple weeks ago, I reclaimed my oldest daughter's late 2013 13" MacBook Pro for myself. She needed an upgrade for her video work so over the winter we got her a fully-loaded 2015 13" MBP. I finally got around to reformatting the old machine, so I've taken it for myself and finally retired the 2011 MBA.
This late 2013" 13" MBP is a great machine, actually. I was a bit jealous of my wife's 12" rMB, but since I do most of my work on the iPad Pro these days, the price of one wasn't worth it. But I'm pretty happy with my "new" and "free" MacBook Pro.
My 2013 27" iMac, however, is due for a replacement this year. I'm waiting to see what, if anything, happens to the Mac Pro line before I pull the trigger on a retina iMac.