There is much debate about whether or not honey should be boiled at all to sanitize it. There isn't much consensus, but I will tell you what I would do.
I would add it at the last 5 minutes of the boil. You don't want to boil it long, and some people don't boil it at all. For a #9 clone, I would use up to a pound of honey for a 5gal batch, but no more than that.