The goal is to evaluate a beer in 10 min or less including consensus as a BJCP judge. In reality with that goal in mind the time should be less, right?
I honestly often find myself bored proctoring the exam because of the extra time. Some judges are incredibly slow, they evaluate the beers, but seem to take forever to get the words on the page. That is why, IMO, the time for evaluation is 15 minutes. I know two judges who have a difficult time writing due to health issues and they still can finish in less than 15 minutes. With practice, and you should practice before taking the exam, you should be able to finish each beer you are presented with time to spare.
As far as the beer changing, the aroma will begin to lessen in most cases making it more difficult to characterize. That is one of the reasons it is on the sheet first. The aroma and flavor may change as it warms, but a 2-4 ounce sample in your hand will warm very quickly. The key is to keep the beer in the weak hand and the pen in the writing hand and never set down the beer. You'll judge faster when you don't have to keep picking up the cup or glass.