Considering the element of time, I would have to say that ideally a good blogger would update at least once a week. However, "ideally" rarely works in practice and that said, I'm of the opinion that not even updating once a week will save you, if it's uninspired crap.
The actual having time to do so of blogging is something that I have and continue to struggle with. It's just so easy for me to fall off the face of updating for months at a time. I'm not entirely sure why. Possibly because when it comes down to it I am a shy person. Possibly because while I'm not short on things to write about, I do run short on the belief that people really care to read about them sometimes. Possibly because I'm somewhat of a hermit and going missing is nothing new for me.
I am beginning to think that perhaps it is good to make yourself sit down and write something on a semi regular basis just for the sake of writing something. Anything. And with any luck once there's some words there, I won't think it's un-postable crap, and will proceed to update with it.