As a rule of thumb, I instinctively disregard all rules of thumb.
1 month to a year -- c'mon, what's the rationale for that, apart from the fact that it makes division easy?
Just be honest with yourself. If you're into the lady, for who and what she is (as apart from being into her for who and what she isn't), then forward, onward, upward. If you're just out to do the Wild Thang, and she's okay with that, then buy the ticket and take the ride.
But to set some kind of arbitrary functional relation, be getting to know someone and liking the getting-to-know, and then go, "Whoa! Now, I was married 18 years, and it's only been 17 months. Call me the Hi-Dee-Ho Man, but I'd better put the brakes on," just flies in the face of all human reality.
