Karen,
There's no pre-determined amount of time required for exam preparations, because everyone learns at a different pace. There are so many different factors involved, such as your ability, the amount of time each day you put in to studying, and the quality of instruction you are receiving. When you and your instructor feel confident about your ability to perform the required figures without flaw and without too much concentration on your part, you are ready to schedule an exam.
Now when I say "without flaw", I'm talking about the basic techniques as specified in your technique book, such as footwork (all your heels and toes in the right places), staying on time with the music, etc. The things that can be described definitively as "right" or "wrong" are the things you want to make sure you have right. Everything else, such as your quality of movement from foot to foot, posture, latin hip motion, lead and follow, arm styling, etc... these are things that we are constantly developing throughout our dance "careers". It is expected that you won't be perfect in these areas, especially at your first exam, so don't wait until you are before scheduling it. Just make sure the basic techniques are correct.
There are two styles of exam: A basic student exam, and a professional exam. The basic student exam requires only that you dance your own part (you don't have to dance as the opposite sex), with your teacher, to music. The examiner will watch you dance, and then give you a score for each dance, along with some comments. The scores and comments are typically mailed to you at a later date, along with your ccertificate (assuming you passed), so that you can go over them with your teacher. And that's all there is to it!
As far as competition is concerned, there is no requirement that you complete an exam. You can enter a competition after only having taken a few lessons. In fact, if you take your first exam before competing, (assuming you take a few months of lessons to prepare for your exam), you will have precluded yourself from the "newcomer" level at the competition, because newcomer level is open only to those who have had fewer than a handful of lessons (I don't remember the exact number... 25, maybe?).
I hope this helps. Your teacher should also have this information, so trust him to guide you through the process. But if you have any additional questions, feel free to ask!
Sincerely,
Jonathan Atkinson
www.ballroomdancers.com
