I just upgraded my phone, I tend to keep the same phone for about 2, mostly 3, years and then get a new phone. Usually that's when it feels more like an upgrade. I don't get those who update their phones every time a new model of their favorite line comes out.
I'm always on a contract so I tend to get a new phone every 2 years. It's good because you get bored with your phone after a while and it's nice to get a new one . I usually give my old phone to a family member, think my current one will be going to my little sister.
I don't have a set timing. As long as the phone is working, I wouldn't try to get a new one.
Well, I do have two phones...but one is a flip phone which I use rarely.