Funny you should ask that now. We live in Interesting Times, demerphq is very busy finetuning his newest toy, demerphq's sandpit, which, I believe, will revolutionize this site, as I expect it to prolongue the lifecycle of threads by quite a large factor. With it, you can easily spot new notes to old threads easily, where they simply got lost in the noise before — people often only looked at new threads, thus, those threads having its root node in Newest Nodes. Therefore I do believe the average lifespan of a thread until now, is about a day.

And this evolution is a Good Thing.