While Villains like to Monologue... Is there a rule about monologue and turns? Like how many words it need a villain to waste a turn in their monologue?

Well, most systems have a rule about how much time in-universe a round is supposed to take. For many systems it's six seconds, since that makes one minute a nice even ten rounds. So you could just time how many seconds it takes to deliver each line of the speech.
Then again, it's a good idea to encourage cinematic moments where you can, and letting PCs and NPCs alike do that thing you see happen in superhero comics where a character can spout an entire paragraph in the time it takes to throw a punch is one way of doing that.

Most of the time talking is a free action