Normally I upgrade each year and the wife gets the 1 year old phone. However, I am not sure she would be interested in my iPhone 6+ so may need to hold onto the phone for 2 years.
I pay $10 a month to my carrier (Telus in Canada) for an add-on that gives me AppleCare+ while also allowing me to upgrade every year, and I got my iPhone 6 on launch day, so I'll be upgrading a year from then.
I also feel like "S" models tend to have more longevity and fix a lot of problems, so I wouldn't have stuck with the 6 for more than a year either way.
I may get the 6S if it's significantly faster than the 6. If not, I'll keep it until the 7. I was with the 5 for two years and it was still a great phone, so two years is pretty reasonable for me.
Before my current job, I didn't have a work-provided phone. I went iPhone -> 3G -> 4S. My current employer supplies phones, currently a 5C. That's enough for me to have ditched my personal phone (ported to a Google Voice account, so it rings my work phone.)
My wife has generally been "upgrade as necessary when phone breaks, or newer becomes hand-me-down." She only got an (original) iPhone when I upgraded to the 3G. She then upgraded to a 3G when she lost the original iPhone. Then upgraded to a (used) 4 when the 3G became thoroughly outdated and couldn't run new apps. Then the 4S when I decided my work 5C was sufficient.
So, from past experience, my wife will likely upgrade in another year or two. I have no idea how often my work replaces phones - the 5C was the first time they offered an iPhone, before that it was Blackberries.
Depends on what features come with the next update. If it is just a memory/speed upgrade, camera upgrade, and few small features here and there then I won't upgrade. After all, I did use my iPhone 5 for two full years.