20 mins is for training dives only per standards. I've logged 15 min dives in a non-training situation. It happens. I just don't have a log book full of 20 mins dives kwim? I make it my goal to have my ow students have the longest possible bottom time (air permitting) instead of rushing thru to get the minimum 20 minutes. I love bringing my group up a good 15-20 mins after another instructor, students with good bottom time, practicing their skills on a tour, instead of crawling along the bottom. If all you ever learned was that dives are 20 mins because of ow training, it's how you start planning your own dives. I know I did. Until I had a great mentor.