Perhaps its my Corporate America experience speaking, but my suggestion would be once per quarter (3 months). I suggest mainly for creating some consistency within the Community, as opposed to "who's the VFM this week?".
Additionally, it would hurt for those that that might consider using this volunteer position as a resume piece (volunteer work is often asked for, especiall in the technology industry), a quarter is a much more substantial and typical length of time.
I do not think 3 months is terribly long, especially if it is a TEAM being "hired", and not 1-2 people in addition to -Hime- and yourself. The duties of a VFM are much lighter, compared to full Admins, Moderators or Customer Support, so there should not be as much stress to deal with, or one would hope. :)