Decision mathematics??Watch this thread
D1 - very easy, not time consuming at all.
D2 - very easy content, practise makes perfect with this, but the exam can be tight on time if you've not practised enough.
Again, depending on exam board, and how much time you'll be giving the modules, I would say roughly 4 weeks for each. The content isn't hard, its the practise that will take the time.
D2 is fine except for 1 chapter, dynamic programming, I find it so hard to understand. In both modules it's easy to make a careless mistake losing lots of marks, so practise is needed.
I got 82UMS on my D1 exam but I didn't finish the paper... Missed the last 5 marks but still got an A on it