Gardner here. My x is "two," like Howard's, and for the same reason. I don't specify a length because then the conversation is about length, not quality. Instead, I say they have to be "robust," like all their participation (online and in class), and we have a conversation about the word "robust." I'm serious.
I actually believe creative linking is every bit as important as length--perhaps even more so.
I also tell them "no bunching," which means "don't ask if you can 'make up' a blog post. Would you try to 'make up' a conversation by standing in the corner and talking to yourself for an hour? The whole point is the essayistic distributed conversation."
Takes a lot of nudging to get this going, which at some point turns to praises for great work, and then the work becomes self-sustaining--but never for 100% of the class, alas.
I've always found my students' blogs to be well worth the dance, well worth the effort, and when they're great, they're inspiring beyond words.
P.S. The "reverend" is Jim Groom. That's his nickname, given to him by Chip German, the foundation of all the success of our efforts (first mine, then Martha's, then Jerry's, then Jim's, and always the whole team's) at the University of Mary Washington.