Pampered hardware will likely last 5 years. Hinges, back lit screens...they can be unpredictable. More likely is that rapid hardware advancement will make a newer version faster, cheaper and better. Absent a hardware failure, you're more likely to face a desire for a model that does more.
However, compared to what many experience with Windows PCs, I expect the productive life of Chromebooks to be longer because they free us of the headaches of registry, drive and malware deterioration.