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.
My wife changes her cases once every few weeks. If she has to dress up to some special dinner, she would put on a case that matches her outfit or hand bag.
My daily goto is a Sena leather pouch. But I have a couple others, like my bamboo shell, that I occasionally switch to when the mood hits me to change.
I've bought too many cases over the years and currently use the phone without one. It was an expensive experience to realize how I like to operate my phone to say the least.