I tried to think of interviews as conversations, and I think it worked out well for me. Therefore, the response time needs to be about the same as if you were asking your friend what they were doing tonight. If they paused and said, uh..., you'd be thinking the same thing. Also, think of it this way: when you ask them a question about their program, like why is the pharmacy building named after this dude? what did he/she do? How fast do you expect them to respond?
If they ask you a question that stumps you, take a few seconds, no more than 7 and think about it. If you can't come up with something, ask them to ask the question again and see what they are really trying to get from you. If you've never experienced something, or don't know, say that. It won't be too big of a deal.