Nobody really knows, because Macbooks have only existed since May 2006. We got ours two days after they were released, and it's going strong. I expect (hope) it will continue to last well past the warranty period, which expires next May. I would also assume that the vast majority of Macbooks are similarly still in service.
By way of comparison, our iBook G3 lasted three and a half years before going kaput (actually it still boots up, but usually freezes up within a half hour or so, making it basically unusable). However, it was plagued by the famous logic board problem which was due to a design flaw on Apple's part. They replaced bad boards up until it was three years old, at which point it was unsupported. The last logic board failed with telltale symptoms about six months past that cutoff point.
Since the Macbook doesn't have any known design flaws as severe as the iBook logic board problem, I expect it to last longer.