Blog Brandz talks about how to avoid blog burnout and mentions that Robert Scoble has stopped his daily frenzy of posting to his linkblog which was taking anywhere from three to eights hours a day. I think for most bloggers, burnout is not an issue but for some, burnout can hit pretty rapidly.
I average about four to six posts a day by spending 30 minutes in the morning and 30 minutes in the evening researching resources to blog about. During these 60 minutes I find the material for the posts and spread them out by future dating them. If I find quite a few resources I will even future date into the next day and take that day off, but still publishing content to keep the traffic flowing.
As with everything, blogging should be done in moderation.