SoCalVal, in a case like that, I want to encourage you to simply interrupt and say, "I can't talk any more, I have to [xxxx]," and just go do it. Even if it means you walk out of the office while she's still talking.
Here's the thing: Even if it's not conscious, a person like that is doing this on purpose. They are manipulating you (even if it's instinctual, and not carefully thought out).
They know the "rules" about not interrupting, etc., and they are taking full advantage of them to cling to the audience that they need.
It's a compulsion--I have some sympathy for them. But that doesn't extend to thinking that they ought to be getting their way.
So not only are they being rude, but there is a part of them, buried however deep or shallow, that knows this.
You know what? They probably don't feel good about themselves afterward anyway. Few people with such powerful compulsions -do- feel good after indulging in them.
So, just say, with a friendly tone of voice, "I can't talk anymore, I have to finish this work/go home." And then go live that boundary instead of trying to negotiate it. Make your actions carry your message: "I'm not listening to you anymore, you can stop talking now."