The number one guide is to be sure that the potential client understands that not all questions can be answered within a given amount of time. As long as the client understands this, everything else is up for discussion.
As the client in my current GF posting, I appreciated that my specialist was clear about what records he would search, and gave a specific time. He also asked me to clarify what I was looking for, as I had been rather vague. I think being clear up front really is important.