At some point in your career you'll be faced with sitting down for an oral exam, especially if you are in Yachting, or going for an MCA licence.
The one thing that came out of my sitting for my final Master 3000t Yacht Exam was that they need to "feel" you are ready, not to mention you have to pass the exam by knowing all of the sylabus inside and out. On average the exam can last anywhere from 30 minutes ( a friend of mine ) or up to 1.5hrs ( myself ).
Time aside it's not really the length of time, but it's your abilty to regurgitate information that they deem necessary, but also answering the questions that they want. Personally, if I've been prepped as to how they might answer / will answer or could answer then i'm going to be better off. They will always find a unique way to catch you off guard by asking you a question that you haven't prepped for, and on that day if you don't have what they want, then unfortunately you will be flying home empty handed.
So knowing what I know now, would I do oral prep..... OH YES!!!!!!