I think it's at least 3 hours, but I've never been able to do more than 15 minutes of it at a time, so don't take my word for it ;)
Lynn doesn't tell you the secret to enduring the honing ordeal.... alcohol. I had to discover it on my own, with the only negative effect that nowadays that doesn't leave me much time for honing.... Catch-22 if I've ever seen one.... uhm yes, cheers!