I used iPhone 4 for 3 years and 9 months (shattered the screen 2 years into the ownership...). Recently got iPhone 5S. I upgraded only because my iPhone 4 died. Otherwise, I would have kept using the 4 for another year until battery wouldn't last more than 3 hours. So I plan on keeping my current 5S until it becomes unusable.
BTW, my iPhone 4 turned on right after I got my new 5S... So I carry the 4 around along with 5S
I've bought every iPhone except for the 5. This time with the 6+, I paid for the phone outright through T-Mobile back in January and it is now unlocked.
Resell previous generation iPhone and use that money to buy the new iPhone (so every year). But this time, I will hold onto the 6 till my contract expires on Oct 2016
Every 2 years when I am eligible for an upgrade. This is my first Iphone, a 5s, and I can't imagine anything better, but in 2 years I'm sure I will be ready for a new Iphone!
This. I find if I keep it in perfect condition with all the original accessories and box I don't have trouble getting 50% of its original cost back. I always put boxes in the loft and keep my phone in a case, so this isn't hard. Sold my 5 to get the 6, will probably wait for the 7 (or whatever 2016's model is called).
I'm on my first iPhone (5, 16GB). It is almost 2 years old now, but it still runs very well. I'm not sure when I'll upgrade, because neither the 5S nor the 6 compelled me to buy a new device.