Every 2 or 3 years it just depends.
I had my HTC Desire S just under 2 years before I got rid of it just over 3 months ago and got a Nexus 4. As I was never 100% happy with the HTC Desire S.
I got no plans to upgrade my mobile again in the next 16 months or so
I had a Sony phone nearly 3 years and only got rid of it due to battery problems.

Darren

Around two 18-24 months usually depending on the phone and advances in technology. I liked the look of the HTC One and liked the screen and needed a LTE phone to benefit over next two years.

Usually upgrade when my contract expires, used to be once a year, then every 18 months and now my S3 is on a 2 year deal. Whaat happens after that depends on the phone that I want next, if the S5 (or whatever) is available

Usually every couple of years or whenever I feel my current phone is outdated. I've had 5 phones in 10 years.

I'll buy the latest phones from Apple and Google when they come out, so probably 2 phones a year.
If you do any mobile development you can't beat testing your stuff on the latest gear.

Just got an iPhone 5s and a just ordered a Nexus 5 and will be selling my old Nexus 4 and have already sold my old iPhone 5

I think my next phone will be another Nexus as im loving my over 3 month old Nexus 4 but that's well over a year away before I get another Nexus. Unless the phone starts to play up before then. For as long as Google keep doing the software updates for it. I will keep using the Nexus 4.

Darren

Well, Google are now stating that they will only update Nexus devices for an "18 month" period now.

They've decided (quite cheekily actually) not to update the Galaxy Nexus to KitKat 4.4, despite the device being more than capable of running it. They can't really use the excuse of "old hardware" for that one, it could have lasted another update, but I guess Google is a business and does want to sell their newer devices.

Of course that doesn't stop the Nexus S and Galaxy Nexus being supported unofficially.

Every 6-12 months usually.

But then i get a new work phone every year so why not.

Got my first mobile in 1993 and I'm now on my third.

wow! That's great.

What are the 3 phones you've owned?

As long as my Nexus 4 gets updates for a lest another year I will be happy. Also my 15 month old Nexus 7. I will be happy if that gets at lest one more update.

Darren

I'm having an iPhone 4S so it's been 2 years. I plan to get the iPhone 5S but I've been waiting for over a month but it still isn't arrive so I'm gonna wait for iPhone 6. I don't think there's much different between 4S and 5S so I will be expecting for the iPhone 6 as the screen is gonna be much bigger, which I do need this improvement.

Every year.

I am going to have a good look at this Kit kat before I update my Nexus 4, i like the way it works and past updates for my last phone changed things I did not like.