I've always heard that you should change your oil every 3,000 miles (5,000 km). That little sticker you get on your windshield after an oil change agrees. Growing up, my parents told me the same thing.
I doubt that you need to change your oil that often to maintain a healthy engine. I suspect it's a ploy by the companies to increase profits.
Is it really necessary to change your oil that frequently to get the most life out of your vehicle?