They will typically fruit off a live oak annually in the fall. However, on dead trees they typically only fruit for a few years. The dead oak eventually gives up it's nutrients and sap and there is nothing left that the hen can use. I have seen hens on dead oaks many times. As far as how many rears they will fruit off a dead oak varies. Could be somewhere between 1 and 10 consecutive years if I had to put a number (pure Guess) on it. But eventually they stop.
I have a live oak I find hens on for the past 10 years and it has never not produced at least one nice one. Good years this one tree can put up a half dozen or more nice ones at the base.