I get asked this question often - and become acutely aware of it when not posting for short periods of time. Last week I was on the road downunder and just didn't have time to get to posting.
For me, the simple answer is, as often as you can and whatever you are comfortable with. Not posting for extended periods of time will definitely cause folks to stray to other things. Will they come back? RSS keeps you on the radar for when you do raise your head - but I am still amazed how many folks are not using an RSS reader.
I'd rather post and read content of some quality than drivel for the sake of posting. So, rule of thumb - shoot for once a week, more if you can.
Thoughts?