I see that there is a 400m in 12 minutes.
Well, not quite. As katdiver said, there are four exercises. For each exercise, you earn points depending on how you accomplish them. You need a minimum of twelve points. So if you do the 400 yard swim in under 12 minutes (or 400 meters in under 13 minutes), that's only two points and you need an average of threes. So you'd either have to swim faster, or make it up in other exercises. The complete set of requirements and scoring fills more than a page in the instructor manual, so I won't attempt to reproduce it here.
edit – decided to attempt a summary after all (metric in parentheses):
Swim 400 yards in under 6 minutes (or 400 meters in under 6 and a half minutes) = five points.
Taking over 12 minutes (or over 13 minutes) = one point.
Two, three, or four points for various times in between.
Snorkel 800 yards in under 13 minutes (or 800 meters in under 14 minutes) = five points.
Taking over 19 minutes (or over 21) = one point.
100 yard tired diver tow in under 2 minutes (or 100 meters in under 2:10) = five points
Taking over 5 minutes (or 5:30) = one point.
Most everybody gets five points on the 15 minute tread with hands out of the water for the last two minutes.
Minimum of twelve total points required.
Don’t stress too much about the tests. If you’re even reasonably capable in the water, you should do fine. I’m a pretty slow swimmer, but still got 16 points overall. Most candidates I’ve taught did a bit better.