Zinn suggests two years. That's alloy bars, probably with regular use. Whatever that means.
I have used 25-year-old Cinellis with no problems (yet), and I have used a 45-year-old steel bar on a Raleigh road bike.
There are so many variables involved that there's no hard and fast rule about how long a bar will last. I've yet to see one fail personally -- but I have no doubt that it happens.