What do you mean by support? If you mean, technical support, quite a while. I think they still offer technical support for the original iPhone (though obviously at this point all warranties have expired so you'd have to pay for any repairs at this point).
If you're talking about how long the latest version of iOS will work for it, that's a murkier question. Looking at the iPhone and iPod touch, the latest version tends to work with two previous versoins of the phone. For example, iOS 4 really only worked with the 3GS and the iPhone 4 (though there was limited support for the original 3G). Though iOS 5 reported will work with the 3GS, 4, and whatever is being announced next week, so Apple may be broadening its support.
Short version: You're probably safe through iOS 6 (assuming we keep up the annual releases). You might need to purchase a new device for iOS 7.