I'll usually be the first in line to tell you when something has run its course and it's time for it to be laid to rest. I think it's similar to Andy Stanley's (at least I think I heard it from him) concept that every once in a while you need to throw out the couch that has been lying around.
Nobody uses it.
It's out of date.
It smells funny.
It's a mess.
Does that apply to a blog?
At what point is it time to close the book?
At what point is it time for something new?