If you update your blog less than once a month, it's not a blog: that's called a "webpage".
Now, if you update it once very two weeks or so, I'd still call it a blog - albeit a slow one (a "slog"? :)). Update at least once a week, on average, and I think you get full marks.
I have to admit, I'm a little surprised with my blog: it's been easier than I expected to update it regularly.
I've been doing it for a few weeks now - its a real commitment. We'll see how long that lasts, but so far its been fun. And Google's Adsense dashboard tells me that the traffic (and click thrus) to my site are worth $0.56. That's some cra-za-zee chedda', yo!
I started to blog because... well, its 2006 and I'm "in the biz", as it were, but its turned out to be a nice place to put all those thoughts that don't really fit anywhere.