I keep all of my Apple products for around 5 years. Sure you can upgrade sooner, but I usually find that they work fine and keep going for at least that long. Most people just like having the latest and greatest. If you have the money, great, but if you can resist the temptation and look after your gear, you really can get 5 years out of most electronics. I will keep my current MBP for 6 before retiring her gracefully. One advantage is that when I do upgrade, I usually see a big jump in performance.