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Well, the longest I've ever had a phone was the iPhone 3G, had that the full 2 years until the iPhone 4 came out.

As for shortest, not sure if this counts, but I got the Samsung Galaxy Mega last year and had it for 4 days before returning it, it was just too big. Other than that one, or other phones I returned during the return window, it would be the Moto X. Now don't get me wrong, the Moto X was an awesome phone and I do miss it somewhat, but I was offered a Nexus 5 32gb for it and I couldn't pass it up. Had the X for about a month.
 
Longest was probably my iPhone 4. That was June 2010 to October 2011.

Shortest was a few hours.
 
Longest I've ever had a phone was with my first smartphone - a Motorola Q from 2006 to 2009. Shortest was probably my HTC One X, lost it after about four months.
 
I usually upgrade once a year, I guess I had the iPhone 4S and Droid X for 13 months each. I had the S4 for about 5 months last year before getting the Note 3, which I have now.
 
18 months at the longest (iPhone 3G), a month at the shortest (GS4)

Usually I upgrade yearly, I plan to sell my current S4 for about a few more months then I'll swap it in.
 
2 years I had my iPhone 3G (Skipped the 3GS)

Shortest I had was my iPhone 5, had it for a month before moving to Android.
 
Longest was my iPhone 4--just short of 2 years.

Shortest was my Nexus 4 (with the iPhone 5 a close second) at only about a month.
 
I'm not sure a few months probably. I basically run my phones into the ground or until they no longer work for me. I expect to be keeping my Moto X until the battery is shot or until it falls apart. Which aside from a nuclear attack I don't anticipate that happening. The only reason I got rid of my Nexus 4 was because the battery would be dead by 4pm and I could upgrade to the moto x without costing me anything out of pocket thanks to my warranty service.
 
Longest: Siemens C55, from early 2003 to the end of 2007. Best phone I ever had, small but with a nice screen, super fast keyboard and bloody loud ringtones.
Almost as long as the C55, I place my Nokia E63 (2009-2012). Excellent phone too, but the keyboard suddenly got crazy.

Shortest: An old Philips in 2002, don't even remember the model; great audio quality, but it literally fell into pieces 2-3 months after I bought it. Build quality was terribly horrible :)
 
2 days - bought a Samsung galaxy Mini 2 as it was small. Bit too small and low res - it gave me a headache and (perhaps predictably) lagged like hell. Returned it for a refund.
 
Longest: Original iPhone, December 2007-January 2009 and then another stint of use after my 3G's screen broke from that point until the 3GS came out.

Shortest: Samsung Galaxy S II. Picked one up when my iPhone 4 shattered and returned it within a few days to renew my eligibility for a 4S. Hated that thing and Android of the Gingerbread era, I'm sure touchwiz on top didn't help.
 
Kept, well I still have my old Sony Ericsson T68i from 2002 in a drawer somewhere. Longest used was a Nokia 3330. I believe I actively used it for 5 years (or was it 6?) until it got stolen (dorm). Ever since I moved to the US with the carrier subsidy model, I've been replacing phones every time the contract is up for renewal.

Shortest use, I've got a ton of GSM unlocked Androids that basically just sit in the drawer that were used intermittently for maybe a few days or so. I try to familiarize myself with Android to see if I'd be willing to switch but so far, I haven't been all that impressed with battery life on all the Android smartphones I've tried. Ever since Ice Cream Sandwich, I like the OS enough and certainly find the muted colors on the default Holo theme more pleasing to the eyes than the neon brightness of iOS 7. However, I'd like to have a phone with stock Android (that doesn't require installation of custom ROMs) that I won't need to charge 2-3 times a day.
 
Shortest: iPhone 5 (Oct. 2012 - Dec. 2013)
Longest: Samsung Exclaim (Sept. 2009 - June 2011)

I intend to keep my Nexus 5 for a while
 
Longest: Samsung captivate and iPhone 4 both for 2 years.

Shortest: Xperia Arc for 2 months, with Nexus S coming as a close second at 2 months and 4 days.
 
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