There is no fixed schedule, but we will get to replenishing the supply for people who have been successfully inviting "soon" (we've throttled signups to keep population/congestion under control as we expand).
@Drakkar - I can invite your friends IRL with my invites if you want. I still have 3 right now. :-D Send me a letter in game if you want to take me up on that, I might not remember to check this thread again.