Last week, Executive Editor David Carnoy posted a poll about how long you should be able to use a smartphone without really needing to upgrade.

That was mostly about how many years you should get before your mobile device's technology is woefully outdated. With digital cameras, though, I'm more concerned with how long the thing will actually function.

My first compact digital camera was a 2-megapixel Nikon Coolpix 950 that I bought in 1999 for a little more than $800. That camera is still fully functioning.

However, there are no fixed-lens compact cameras in that price range, only digital SLRs and interchangeable-lens cameras. Currently, the compact market taps out at $500 and those are the high-end megazooms that, sadly, I wouldn't expect to last more than three years. They're just not built for long lives of regular use.

I'm convinced that current digital point-and-shoots priced less than $300 will work for a maximum of three years before dying. Basically, for every $100 you spend up to $300 you'll get one year of regular use. After three years of use, you're shooting on borrowed time. Spend more than $300 and chances are you'll have a paper weight before the camera will earn out.

It's a safe assumption that the more someone spends on a camera, the better they'll treat it, too. That's one reason I would expect digital SLRs to last longer than an average compact camera (at least the midrange-to-professional models). The ones I'm on the fence about are interchangeable-lens cameras since they have a lot in common with point-and-shoot cameras, but seem to have higher-quality construction.

Of course, this is all just my opinion based on testing a lot of cameras and reading a lot of user reviews from disappointed shoppers who weren't thrilled with replacing a camera so soon after it was purchased. What are your experiences or expectations?