Just one extra thought - three months as the time limit, but an option for four if necessary. For instance, Eru has stayed on a bit this time around because us other admins asked her to. With the enourmous amount of invites being processed, the fact that Gimli has given up his admin status, and Griffy has problems accessing the panel, it's just to much for only two admins.
As for requirements to become an admin - I would say something like 1-2 months of active posting. The only real reason I can see to limit it (as the voters will not vote for people who have only posted once in six months, or only post one-liners) is that people should get to know the board and the system before they can be the one to push the buttons