Believe it or not, you can practice a speech too much. If it is too practiced it can sound forced and unnatural. You are aiming for a natural sounding you talking about something you are passionate about. The rule of thumb is that you practice it often enough that you can remember the key points and are relaxed. This might be 6 times, it might be 20 times. It’s all about how comfortable you are with the speech. Hint: practice in front of a full length mirror so that you can see what the audience will see.