One question that many speakers ask me is, “How long do you practice your speech?” My answer, “however long it takes!”
Winston Churchill once remarked that he practiced one hour for every minute of actual speaking time. In my case I practice in short blocks of time over a space of many days or even weeks. I personally feel that you can never practice too much. The more I practice, the more natural and comfortable my presentation feels to me.
Secondly, this extra practice time allows my mind to think of different words or phrases to use to say the same thing. Even different ideas will come to my mind. How many times have you after you spoken with someone said, “Gee, I wish I would have said this rather than that?”
Practice lays the groundwork for your speech. That is why you can never practice too much!