I've had two PC laptops so far (am on my third now) and both "survived" for about 3-4 years. In the third year, there were first signs of trouble, like CD drive breaking down or the screen not turning on sometimes.
BUt i guess it depends on how much u use them and how much u carry them around (as opposed to just having them in one place most of the time).

It depends how you use a laptop, I had a laptop for 8 years without any trouble, but didn't use it ever on the internet, but I forgot to use a power surge protector & that ended it. As it only smoked & didn't catch fire, it wasn't covered on the insurance either!