Moderator pro tem nominations generally happen about a week into public beta, and are influenced by the community (according to the blog post).
Just how early should a new site start their meta question for pro tem nominations (or indeed, the other 6 beta questions)? In private beta? or when they graduate to public beta?
The general idea that the blog post gives me is that it should be as soon as possible, but private beta feels a little early, with only a handful of meta users