The best thing to do would be to time your performance with the microwave or something. Like set it for 30 minutes and see how far you get before the beeper goes off, (if you can hear it

). I looked up some times for you though.
Bach Prelude and Fugue, no. 12 book 1 in midi form (so in real life it probably is a bit longer) was 4:22.
Schubert Sonata in A Major, I found one performance at 17:28 and one at 21:15.
And if your Debussy is the same one as la cathedrale engloutie (I don't speak french, so I don't know), then I found a midi at 5:30, and one at 7:48. However, I also found a live performance at 6:47 which is more reliable than a computer generated midi. So........
4:20 (probably a bit longer remember) + 17:30 + 6:50 = 28:40 which means you need an extra 1:20. Take 10 seconds between pieces and before starting which will give you an extra 30 seconds. 1:20 - :30 = 50 more seconds needed which I highly suspect will be made up in the Bach LIVE. The problem is, this time is
WITH repeats. So, what you need to do is time YOURSELF. But you really might need to do that repeat as much as you don't want to. Especially in an exam where you might play a bit faster than normal due to nerves. What does your teacher say? What do the rest of you say now that you have an idea of how close the time is?
(I'm not commenting as with RCM it's the opposite. We have a time limit to stay under not get over).