I have had my Jolla as my main phone since the beginning, and using it for a year has not made it any worse. I am still happy with it and no hurry getting the next one.
I'd say Q4/2015 would be a good time to replace my device, unless something happens to it!

I consider 2-year cycle in changing phones the absolute minimum, it makes no sense to rush out and buy a new one immediately when your current device can still do everything you need it for