Well, I'm a bit of a control-freak about my updating schedule and keeping it regular, so I've got a pretty huge buffer. Not counting the chapter-covers, which will be going up as separate updates, I've got a 52-page buffer, and I'm posting twice a week.
So I've got about a six-and-a-half month buffer going at the moment. Since I'm doing a bunch of stuff OTHER than my comic, it feels nice to have that. I'm planning on taking a week-long break between each chapter, too.
... I might be working on making that buffer even bigger as we speak. Better ahead than behind!
Generally speaking, having a buffer is a good thing. You don't have to go overboard the way I did (and gosh, did I ever), but having at least five or ten pages, depending on your updating-schedule, is really nice. You never know when something is going to get in the way of your drawing-time.