Unlimited technically. Each infraction gives a certain amount of points. The amount of points determines the ban length and how long the points take to expire (the points can remain active even after the ban has been lifted, meaning another small infraction will give an auto ban).
The worse the infraction the longer the time period. 50 points is a 1 year ban. Supermods/Admins can perma ban an IP. Regular mods can perma ban members that registered in the last 30 days (which is great for all those link spammer bots).
Although, obviously if someone keeps posting porn/racism etc they'll be perma banned quickly.