I'm sure you'll find someone who will tell you that their iMac still runs after 6 years with no upgrades, that doesn't mean it's a good idea.
3-4 years is pretty standard in my experience especially when you're using your computer a lot for high demand programs like Photoshop. Doesn't mean you can't go longer but eventually the hardware is going to wear out or it's going to become so slow it'll just be frustrating to use. That's just the nature of technology - it doesn't last forever.