“I have only made this letter rather long because I have not had time to make it shorter.” — Pascal

“It usually takes more than three weeks to prepare a good impromptu speech.” — Mark Twain

“If you want me to talk for ten minutes, I’ll come next week. If you want me to talk for an hour I’ll come tonight.” — Woodrow Wilson

Also:

Harold Macmillan: “What are you doing, prime minister?”

Winston Churchill: “Rehearsing my impromptu witticisms.”