Usually 3 years or until it breaks. I don't really trip off having the new phones unless it a huge leap in the technology like when everyone moved to touchscreens.
I had a 2 year period that I didn't have a cellphone and it was so lovely
If I could do that again I would but between my job & my fam its impossible.
I don't see any phones on the horizon that I really want, need or that can do anything most phones can't do already