Use the premise that 2000 words will last approximately 10 minutes talking time. So for an hour’s speech you will need approximately 10000-12000 words. It may seem a pain to write this much for a speech, but it is a good guideline for the amount of information you will need. After some experience this will come naturally and you will not need to be so pedantic with the preparation. It is better to have too much information than finish a lecture ten minutes early, as if you are a new public speaker you will not want an extra ten minutes question time. Also if you are being paid to speak they may feel they have not got their money’s worth.