Life Expectancy of a Good Post

I wonder what the rule of thumb is for the amount of comments on a post. I have 1 or 2 blogs that I keep up with (obviously mine is not one of them). One of these blogs usually averages about 3-4 comments per post and I almost always enjoy both the post and the comments. However, one of yesterday's posts currently has 18 comments. It was great up until about the 11th comment when things took a turn for the worse. It seems once there's a bad apple in the post, it's pretty much over.

When I stop to think about it, I can't think of many posts across the blogosphere with multiple comments that don't eventually degrade into some verbal barrage. So, I wonder what constitutes a good post in regard to comments? Less than 10? At what point to numerous comments ruin it?

Of course, in regard to my blog, I might as well be asking if a tree falls in the woods with no one around does it make a noise. But... that's my fault, now isn't it? Semi-annual posts don't exactly demand regular readers.