Honestly, not much. Maybe 5 or 10 minutes tops. After I publish a new hub I will usually post to a couple of Facebook groups, Google+, Tweet it and then create a backlink on my blog. That's really about it. I do have some good friends who backlink to my hubs from their blogs as well. Occasionally, usually months later, I will go through some of my older hubs, spruce them up and then promote them again. I am curious how other promote their hubs though.