Promotion's aren't based in time I'm afraid, but on trust. There are only three ranks (four if you count 'not allowed on') which are player, mod/op and administrator. You could be a good player who builds amazing things, but if I don't think you're sensible enough with the powers a mod/op gets I won't promote you (as you could quite literally lay waste to the whole server).
Now that's not to say 'if you're not a mod/op already I don't trust you', sometimes I'm just a little slow getting round to upgrading someone, and sometimes I just like to see/interact with someone a little more before I make up my mind.
As it stands I'm thinking of making an 'advanced player' category (maybe not called that) which will give you access to all features that are less powerful than worldedit... you'd be able to do a lot more, but not have the server destroying powers that are WorldEdit.
What do people think of this?
~Longeye~