Until now I only bought a new one when I really needed it.
My first one was a used black iPhone 4 (16 or 32 GB, can't remember) by the end of 2010. But I sold it very fast again because I did not really need a Smartphone at that time. Internet on such a small display was new to me and I didn't like it at all back then. So the iPhone and the rates for cellular Internet were too expensive for my purposes and I switched back to my old phone and for music I bought a new Sony MP3 player. I also did not own any Mac at that time and hated iTunes on Windows.
So my real first iPhone then was a used white 5 with 64GB in 2012. It replaced my old phone and the music player. I also got my real first Mac that year.
I upgraded to a new golden 6 with 128GB six weeks after it came out last year, mostly because of the slightly larger display and because my whole music library alone was more than 64GB at that time, also I liked the new design, especially the golden color.
As I saw the 6 plus for the first time I was shocked about the size. I actually planned to buy that instead before I saw it in real, but then I changed my mind immediately in the store.
I might keep the 6 until it breaks or is not supported with iOS updates anymore. Maybe the 7 will be so impressive that I have to buy it. Who knows... I will see then.
But I surely skip the 6s. Thought about a 6s plus because of the larger screen. After nearly one year of using the 6 it appears not to be that much oversized anymore. But it's still too large for me carrying it the whole day around and too small to replace the smallest of my MacBooks and the recent iPads are also not powerful enough for that purpose.
I'd rather get an iPad Pro or Air 3 (depends on if or when it will be released) in addition to my iPhone 6 and only take it with me when I really need a bigger screen. Then I don't need two MacBooks anymore and only keep a big one for home usage.