I try for everyday. The only reason I may not post daily is if I'm tired up working on other parts of the site, but normally that ends up in the forum anyway.
I try to post at least five posts on my forums each day. Five posts, one post in each node. Spread it out to help the forum look more active on the index page. 🙂
I would post a lot more often if others would respond to the threads. I don't want it to seem one-sided on the forums, so I may start working on articles/blogs for the time being.
A forum owner should engage with members at least once everyday. The little time he has to come online, he should answer queries and take part in few conversations.