Mostly, I blog once a week, normally on a Thursday or Friday evening. It seems that's the only time I have to get to play (as my wife likes to refer to it) with my computer.
It isn't that this is the only time that I think about posting though. Whenever I get an idea or see something worth commenting on, I'll make a note of it in a text file I keep on my desktop.
Once a week I'll email all those ideas to my home address so that when I get the chance I'll write them up. And if I don't write them up straight away, I'll start a draft and finish them another time.
Why do I blog? That's currently a draft that I'll finish another time.