How long before a speech should you begin practicing?

The longer, the better! You want to really know the speech, not just memorize the words. If you wrote it, you can probably begin studying a bit later, since you know what the speech is about already.

You should begin praciticeing a week or so in advance, ive found that in doing this you have time to get tone and the nervousness of reciting the speech out of the way, and will also have the time to make any last minute word changes to make you sound smarter and more sophisticated and professional.