It's always "when".
Sometimes, "when" happens early on, sometimes a CEO manages a 20 or 30 year career, and then "when" comes much later when a successor is named. Other times, "when" comes in the form of a sale of the company.
But, it's always when, never if. Part of the CEO's duty is to prepare for the when, to make sure it happens in the right way at the right time.
There are many business books that cover this topic, but "Good to Great" is a recommendation if you haven't read it already. There are a few good examples of cases where the CEO was the entire company, which in the end is a sign of poor leadership and management skills.
All of the people lauding accolades on Zuckerberg sort of surprise me. He has done little more than ride the wave of social networking on a chance. Don't get me wrong, he has done better than others, but so far I have not really seen any indicators of visionary leadership or great forethought from him. And back to my first comment, his recent interview doesn't do anything to enhance his image.