The poll has a case thats not exclusive to the others, i.e., When it Breaks.
If I have equipment failure, I replace it immediately, as my profession relies on computers - however, if its working (or the failing component is easily replaced) I like to try to get about 3 years out of a machine.
If were on a tech cycle where theres a major modular/replacement component that will significantly extend the life (for example: SSD), then a machine might last even further.
Funny enough, every one of my Winders© notebooks were replaced early due to some kind of failure, often times, a physical issue (cracks, busted hinges, power connector loosening from the MB, etc).
Right now were using a 2010 MBP 13, with 8GB RAM, and a 256GB SSD, and a 2011 MBP 15 with 16GB and a 512GB SSD. Both machines have more than enough horsepower to get the job done (mine is used for fairly heavy duty development including running multiple VMs) and neither (er, knock-on-wood) have any problems, defects, or are showing signs of breaking.
You mentioned the retina iMac? That's the computer I'm upgrading to.
We were walking around the Town Center yesterday - nice open air shopping center, and the weather was beautiful - anyway, popped into the Apple store to check out the 5K iMac. Goddam, that display is fantastic. No doubt about it, if I was at all considering a desktop, that would be my next machine (maybe even sooner than my usual upgrade cycle).
I only replace early if something can be justified as a time saver [...]
Yeah, the same here. If I cant perform my tasks with reasonable performance, and theres not some way to upgrade my existing machine (thats a reasonable cost vs. selling and buying new), then Ill consider a new machine, but that hasnt come up for me yet, machines seem to break or just keep running as Id expect and are upgradable (like a TB hub to add some USB 3.0 ).