I'd have to say more than 9-12. a CGSO would give you 9 IP, a 0.00 ERA and likely a very low WHIP as well.
On average, you'd expect an owner to pitch his whole staff (5 SP's)in any given week, some pitchers might go twice. I wouldn't want to put the minimum at the 45 innings 5 games would entail, but I think I'd want him to pitch at least 4 games, or 36 innings based on a 9 inning game. Since he probably won't be using middle releivers too often, you could maybe say 8 innings pitcher per game, bringing you down to 32 IP. 30 seems like a reasonable minimum to ensure owners stay active and submit a competitve line-up every week.
* Steroid induced record