I just tested this, and I think Martok is correct.
If the flood check length is 30 seconds, then any user cannot create more than one post every 30 seconds.
I just tested this by creating a spam phrase and then making some posts containing that phrase as a test user. I can only make a post every 30 sec0nds, no more. So it all indicates that you don't have this set.
At the very least, for all new users it should be 30-60 seconds. You could then promote users after X number of posts and/or X number of days registered to a group that isn't as strict. That would certainly give you the desired effect.