There are very few inspirational quotes on public speaking. Most seem to have been written when attendance at church or political meetings was more common; people had the opportunity to be bored and speakers to be boring.
Make sure you have finished speaking before your audience has finished listening. (Dorothy Sarnoff)
Possibly good advice on not speaking too long, but better advice is to be interesting for the entire time you are to speak. To see your audience flagging after 15 minutes when you are to speak for 30 minutes is your problem, so be prepared for it.
There are only two types of speakers in the world. 1. The nervous and 2. Liars. (Mark Twain)
Never admit to either.
All the great speakers were bad speakers at first. (Ralph Waldo Emerson)
Don’t agree. No one need be a bad speaker, even at their first attempt. Take the advice given to a tourist asking how he could get to the Royal Opera House: “Sir, you’ve got to practice really hard.”