In SO help center I found a section Does the ban last forever? which says about question bans that: "If you're unable to improve your existing questions, you'll get the chance to ask a new one 6 months after your last question. If that question is positively received, you may be able to continue asking questions; if not, then the ban will be reinstated."
However, randomly trying some SE sites, I did not find the same sentence in help center. (I tried Ask Different, Drupal, GIS, Mathematics, MathOverflow, Parenting, Skeptics, SuperUser, ServerFault, ...)
So are the rules for SO and other sites indeed different? Or is this information simply missing in the help center on other SE sites?