You should only upgrade when you need to. Chances are good you won't wait that long or be that sensible.
So, basically, to me, it comes down to a few "tipping points" as to when "upgrading" (more realistically you may be sidegrading [e.g., crop to full frame], or expanding [e.g., adding mirrorless to a dSLR rather than replacing it]) has been worth it for me. You will find these reasons not unlike those that decide when it's time to purchase a car. In order of most-sensible to least they are:
When your current camera dies and you have nothing to shoot with.
[Personal "worth it" metric--your eventual value may vary] When the sum of the new camera body's tier and generation counts above/after my old one is at least 3, and my old body is getting near that 100k click mark.
When your frustration level with your current gear makes letting go of money you've set aside more than worth it--and you've done enough research to know how the new gear eliminates (or reduces) the frustration. [May involve discovering previously-unknown/unresearched tradeoffs after purchase that may equalize frustration more than envisioned].
When you can find a rockin' deal (usually used or refurb)
Just because you **bleep** well want it that bad.
However. Keep in mind that nearly ANY other piece of gear you can purchase will hold value longer and better than a camera body. Lenses, flashes, and support gear can be used across multiple cameras. They tend not be replaced every 1-3 years, and they also tend to hold value better on the used market. Camera bodies depreciate even while they're still new.
And, in addition, experience and training often count for more when it comes to image quality than any piece of gear. If the goal is to get better photographs, then your money may actually be better spent on airline tickets, or a seminar, or books and training videos than on any camera body.