You'll always get a response and if they're smart, you'll get responses that you'll likely be able to use. I always ask for 15-20 minutes of their time. But make sure you get up to go when the time is up. They'll likely stop you especially when they're on a roll, but attempt to keep your word about the length of time for the interview whether on the phone or in person. Listen actively when people talk.
Most people know what they want when they ask a question.
The trick is to listen to what people are really saying, not for just what you wan to hear. Active listening is a several step process. First, be prepared ahead of time with several open ended questions. This way you entice them to talk. Second, make a sincere attempt to understand the main point of what's being said and take notes.