It's not long, for a human being who legitimately wants to be a member of this forum -- I just have to lock the PM system from new members for a short time, just enough to make all potential spammers lose interest.
Spammers are in a hurry to advertise their "crap" far and wide, and quickly. They usually have no patience.
If I advertised how long you had to wait, spammers would take notes and come back in exactly that much time.
I want to maximize the discouragement for spammers, to the point that most of them give up altogether.