Ooo-Ooo, I know the answer to this question. Pick me! Pick me!
It's not a question of time. It's a question of need and qualifications. When LF staff determine they need a moderator for a specific forum they evaluate existing members for their contributions, abilities, and level headedness (at least that is the rumor anyway). Once they select a member from the the pool of candidates they will invite that individual to become a moderator.
The important point to all this is they'll ask you. Don't ask them.