You want to learn from customers, but it’s also good to respect their time. I usually keep skype calls or interviews to 15 – 30 minutes and let them drive the conversation. I prepare some questions, but they are not more than fire starters. All I have to do is steer the discussion and ask keep asking why, when, how and what.
Try to see talking to customers like going on a date. Try to stay positive, make a good impression, and let them do most of the talking.