Most of our meetings last 45-75 minutes. The shortest one was 35 minutes. We had already sent the questionnaire in several weeks earlier. Some ask your son to complete a personality quiz and those take a little longer. "very casual" is correct but don't get me wrong, the scouts are doing more than putting a name with a face. If you say your son is 6'0", 185, they are, at least subconsciously, making sure that he's not 5'9", 155. They are probably reading body language of you and your son. Although they do not want your signing # until May, they are making a preliminary decision as to whether he is someone that they ought to allocate more time to watch in the Spring. Take notes on each visit. You would be amazed that after a couple of weeks, you forget little tidbits, etc of what the scout may have told you.
Don't get stressed about it at all. Let your son answer the door, look him in the eye and grip his hand with a firm handshake.
As has been mentioned on this site many times, enjoy. After a couple of visits, you could almost ask the questions for the scouts. After the scout leaves, spend a couple of minutes talking amongst yourselves about questions that could have been answered a little differently by you and your son.