One to two years, depending on money and how well it continues to work for me. If I had unlimited income? Every couple of months so I could play with all the toys.
I usually get a new phone after 1-2 years (8 phones since 02). Briefly tried going from G2 to 2014 Moto X, but came to the conclusion that G2 is a better phone. Now I'm waiting for G Flex 2.
Before getting a smart phone it was often as my phones broke alot. I upgraded to a iphone 4 and replaced it 1 year later with a 5s that i had 1 month lol that is when i got my gs3. I love it and will keep it as long as i can.
Rarely. I have too many other toys I want so I cant constantly keep upgrading my phone like some do. Not knocking those who do, just for me like I'll probably keep my Note 4 for almost 2 years.
I get a new one about every 18 months, however I did grab the Galaxy S5 off contract. Kept it about 4 months then grabbed the Note 4. I'll probably get the Note 5 when it comes out if it has some awesome features "I can't live without" though.
I'll buy/sell phones about every 2-3 months. I usually get one "brand new" flagship phone per year, usually on-contract, the rest are off of swappa (I try to wait until last year's model is under $300, then do the buy-sell game to cheaply try out different devices). Sometimes I go back to a phone I used to own if I get nostalgic. Still haven't found a perfect phone yet. I guess I'd want an unlockable bootloader 2013 Moto X with the 2014 X's internals, with the Note 3's (or Note 4's) camera.
It's my only hobby
In the past 12 months...
Nexus 5 (two or three times)
Note 2
Note 3 (twice)
Moto X 2013 (twice)
Moto X 2014 (pure edition on AT&T and 2 different moto-maker versions on verizon)
Droid Turbo
Oops...I guess that's 12 phones in the past year.
My wife could care less about what phone she has (other than it has to be BIG and have a pen...so a Note). So I also get my fix upgrading her phone. She's had the Note 2, 3, and 4 in the past year.
I'm very conservative when it comes to expensive flagship phones, I mean..
When I'm spending £500/$700's on something, I really do like to get my 'moneys worth'
Granted, that was before I started working for a gaming company doing mobile games, so I may upgrade more often now, but I wanted to ride out paying off the phone if I could. I was down to $5 owning against the phone when I gave in and upgraded.