Answer posted to: https://security.stackexchange.com/questions/60691/length-of-passwords-that-are-rainbow-table-safe
Moderator deleted it on the basis that my answer focused too much as a response to another, and didn't address the actual question (which it did).
I have since heard back that if I modify the answer to more directly address the question, it'd be considered for undeletion. No time for that at present, sharing this in case it benefits anyone else.
As I lack the reputation to post a comment to Mac's answer; I just wanted to point out that the emphasis on length only applies if a brute force attack was carried out to exhaust the Key Space character by character.