Usually I'd like them to be a member for at least a month, but if I'm just starting out a forum I'll usually just leave everything to myself. Most of the time an Admin can Moderate everything for at least the first 50 members.
If I desperately need staff with a few members, then I'd probably give out some sort of test to whoever's willing to take the position. I'd then use their responses to decide, not their activity. In that sort of scenario, you probably wouldn't have anybody to choose in the activity pool.
The current 2 CT members and my Associate Admin on my forum have been there from the start, so I have no issues with them. My RPG Admin started as a RPG Mod a month or so after he registered. I usually wait till it's needed to make staff, however, in some cases, I might make 1 more staff than really needed.
If it's a package or review position than pretty much straight away. Same sometimes goes with moderator. I have to know the users experiance and a bit about the user before they become admin.
IT depends on a lot of variables here Brack. If I need immediate assistance than I will usually hire right off the spot, but usually for my forum you need to be an active member.
I need to see the applicants ability on the position they requested for.. if they don't meet basic requirements I'd rather run it myself. For admin assistance I would hire someone I have known for long that I can trust.
I wait awhile. I have trust issues and you don't always know what a person is going to be like as a staff member. And then even if someone agrees to be your staff there is no guarantee they'll be active afterwards.
I think it's best to only make staff of active members and I made staff of members after they had been a member for at least a month on one forum, but at the time it was because I was going to be offline for a week.
If you are looking for a moderator you should just PM a dedicated member (assuming you have any) who you think would turn into a good moderator, then ask them if they are up for the job. Just use your judgment. Its easy to tell a good egg from a bad egg.
I agree with theezy. I have a similar issue, in need of a mod, but I also ask for any experience at all. I mean, you don't want to hire someone with no understanding of moderating, even though you trust them. (but, that can contradict itself, if everyone thinks the same)
Where possible, I like to hire staff from the forum member base. Hence, I don't usually hire them, until the forum has grown to a certain size and I've identified strong contributing members.