Interrupting is almost always rude, gnu, except in the case of the rare person who never ends a sentence. I have run into them once or twice.

One night I was sitting in Lucy's kitchen while Ann was talking. I noticed that Lucy looked extremely tired, and I decided that when Ann came to the end of a thought, I would announce that we were going home.

Ann then talked for 20 minutes without putting a period on a sentence. I know because I watched the clock. After the twenty minutes I barged in to announce that we were going home. Ann was clearly offended at my interruption, but to me Lucy's need was greater, and I don't apologize. (I doubt if Ann is even aware of her bad habit.)

I recently read a story by Patrick McManus (American humor writer) where his narrator produces one sentence which lasts four pages. It's the same type of thing.