Ideally, once a day. I think this is very difficult though, and I'd prefer to maintain a high standard in my posts rather than dilute quality for the sake of keeping up a ratio of posts/day.
Blacknight got it spot on when he said "Post as often as you have something meaningful to say". At the moment it has been six days since my last blog post, which is quite a long period for me, but I haven't had anything interesting come to mind to write about, so I'm not going to force it. And my last post went down very well, so I don't mind leaving it there for a little longer than usual.