Depends what you're trying to do. I read a case study by a guy who was trying to rank a brand new site at #1 for the term 'live help desk' (it was actually a challenge that someone put up on here. Don't the link).
He updated his content every day for 39 days to get to the number one spot. Obviously, there was a bunch of other stuff he was doing but he credits a lot of his success to this.
If you just want to keep your readers happy, I'd say once or twice a week. If you're pushing to get to #1 for a particular keyword, then I'd suggest every day.
L.