I think it is the quality of the practice rather than the quantity of it that matters. If you can make practicing enjoyable then that is even better. So how do you make the quality of your practicing count and be enjoyable?
1) Don't make your practice sessions too long. Shorter sessions more often, is better than a long single session. You will concentrate better and it won't seem like a long boring chore.
2) Don't do the same routine every time: i.e. exercises, then scales, then pieces, then sight-reading. Concentrate on a particular area i.e. a hard section of a piece, or the scales you have the most difficulty with.
3) Don't leave all your practicing to the last minute before the exam. You want to be in the position that you won't be nervous because you haven't practiced enough. Being nervous because you are performing is natural but being nervous because you have haven't practiced enough is destructive and it will almost lose you marks.
4) Perform your pieces in front of family, friends etc. Apart from giving you confidence, it will allow you to point out your weaknesses in a way that lone practicing doesn't.