As of five years ago, it was just a little over three years. They're either getting fired, or they're getting hired somewhere else. In the last five years boards have tried to lengthen the time. In private universities it's probably 10 or 12 years. In public universities the time is shorter.
Various groups define success differently.
A donor who's going to give you a lot of money may define success in ways that others don't. Higher ed has undergone a lot of change in the last decade. Between 1900 and 1950 colleges grew by 16 percent. Between 1990 and 1998 they grew by 18 percent. I was a student, and I remember someone saying you ought to work on a master's degree.