Q: What is a fairly reliable metric to know that your blog is picking up in popularity?

A: When the number of spam comments you receive goes up noticeably and they occur with regularity.

New Q: How long is this metric valid?  Is it possible it can max out?

Corollary Q: How long are you willing to put up with this metric?

Second Corollary Q: If you bring this up in conversation, how long will it take for someone to reference Monty Python?