Depends.
Had my first Mac laptop for 6 years. Had my second for 4 years. Just got a new Macbook Air...not because of any defect in my last Macbook, but because the Macbook Air is just too convenient for traveling.
Still kicking myself over selling my 1st Mac (that lasted 6 years)...but yeah, nothing beats the Air if you're a traveler.
FWIW, I think PC products have to be replaced more often. They're just more susceptible to poor performance (and viruses)....maybe that's some of my Mac bias peeking through, though.