I don't have a particular formula that I use... it's more organic than that. When I first launched my new community, I moderated everything myself. When I started to feel that it was more than I could keep up with, I added another moderator. When we felt it was too much for us to keep up with, we added another.
Of course, this also depends on how much moderation you want to have. For example, if you want people who are very active in discussions and answering questions, that takes more time. If you want moderators to approve all content before it goes live, that takes more time. If you want someone who simply reads all content to make sure it fits within your community standards, that doesn't take as much time.
In my communities, I am the only full-time person devoted solely to the communities and the management of them. On PC World, we have 3 volunteer member moderators that do the majority of the reading and moderation. On Macworld, the editors perform community moderation duties as part of their editorial duties. All of them are part-time.
I hope that helps!
I think Kellie is right on wrt "how much moderation you want to have". In the UK Jive customers do a lot more "pre-moderation" (every post is read before being uploaded). BBC does that with Have Your Say and they have at last report 6 moderators handling tens of thousands of comments per day.
In the States there's a bit more liability assumed by doing that so US Jive customers rely on 1) automated filtering/intercepting of content and 2) community policing (like the Abuse button here) with to send potentially offensive content to moderators.
There is a risk in doing post-moderation too -- something truly offensive and awful could be on your site for hours before you or your moderators come across it.
I do generally recommend post-moderation (as opposed to pre-moderation) though. When you are approving every post before it goes live, I think it stunts the conversation. Instead of a natural flow, all of the posts you've approved go live at the same time. This makes conversation happen in batches, which doesn't make for very good conversation.
Also, at least here in the US, it is not the norm. Most people have used some sort of commenting or message board system, and if they don't see their comment right away, one of two things will happen -- 1) They will post it a bunch of times, wasting your moderator's time, or 2) They will get frustrated and leave. Neither of which is a good solution.