Whenever work decides to upgrade 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 Then upgraded to a (used) 4 when the 3G became thoroughly outdated and couldn't run new apps.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.