I update mine about every 1 to 1.5 years.
I use mine mostly for work: voice, email, calendar syncing, and document creation and editing when on the road. Also, I use it for tethering to laptop for internet access when traveling and as a web browser both for work and for pleasure. Strictly for pleasure, I use it to stream the broadcasts of MLB games when I am at work or otherwise cannot watch the games, and I use it to read some of my kindle library when waiting. It has a camera/video camera, but I use it very little. I don't play games on it either.
AT&T apparently likes me (or just wants to keep me as a happy customer because I have an expensive plan), so they always have let me upgrade even though I'm usually well before I'm technically eligible.
Edit: I selected "about1 year or less" as that was the best fit