I've had the same Incase Slider on my iPhone 4 for the last 2 years. Never changed it. Its survived a few drops and has a couple cracks but I will be upgrading when the new iPhone comes out so I'm holding off on getting a new case.
Guess I am the odd one here. LOL. I sometimes change my case daily. Yes, I have THAT many. I just enjoy changing the look of the phone with a different case. Right now, I have an Incase Hammered in Iron on my 4S. Really liking the look and the feel, so I haven't wanted to change this one.
I've had about 3 cases that I have used on my 4S. I really should just go naked because I usually just get the clear cases, so it shows off the white of the phone. I refuse to put any bulky cases on it since I feel it takes away from the design and feel of the phone. Currently, the Incipio Feather clear case is fitted on the phone.
Somewhat often I guess. I've had my 4s since April and am now on my 4th case, and it's my favorite so far, Speck grip. Plenty on eBay for 15-25 dollars, compared to 35 for a retail pickup at Apple or wherever. It looks bulky but is actually light and has a confident grip.