Only when I need to.
I'm very happy with my 5s and IMO the 6 is too big, so if there isn't a 6s mini next year I may keep on with my 5s until it's unusable.
Upgraded my ipad from the 4 to the A2, so 2 years, but only because I had the 16gb wifi and needed cellular and more storage. I hope to go longer with the new A2, ideally 4 years.
The new tech is always tempting, but think of all the other things you can do with that $$$.