From what I've heard, Ben generally goes out and visits their sites 4-5 days at a time ... about the same length as my own site visits these days.
Throughout the process, he is Bill's sounding board for different ideas, i.e., do you think these two holes would be better or those two? What about a bunker there? And of course he sometimes suggests stuff of his own ... but it's suggesting, not commanding. He respects Bill beyond words and he respects their crew a great deal, too, and he doesn't ever want to "pull rank" on them. But he does have a lot of input, especially around the greens.
My impression is that Austin Golf Club is mostly Ben's design. Bill told me since it was a place Ben would be spending so much time, he thought Ben ought to make most of the decisions there. And when Ben showed me and my son around the course a couple of years ago, he did ask me more questions about specific holes and what I might have done than he usually does.