The Best I have achieved is a two hour-long real-life debate on the Balrogian problem. I can't remember weather we said the wings were real or metaphorical in the end, but I do seem to remember annoying said person by using book quotes whereas he had only seen the films!

But we still managed to go on for two hours.
I have had 4 hour long conversations over MSN with friends about Bombadill, Feanor, Turin and others, but nothing that immediately springs to mind as being more than 4 hours long.