The first time you break a rule, you will be told about that rule. Then, you will be warned if you break it again. If you break it a third time, you will be given your final warning. The fourth time you break that rule, you will be blocked/banned. The time depends on the rule and how you broke it.
NC-17 fanon results in a one week ban, then one month, then an indefinite ban.
This is very similar to Wikipedia's "three warnings and then banned" policy.