@Sasha what I usually do is this (not something we agreed upon, just something I use as a guideline for myself).
-1st or minor offense - 3 days
-2nd or major offense (insulting admin/players or multiple offenses) - 1 week
-3rd or re-occurring offense 2 weeks
-4th offense --> permanent ban or at least a month
Of course there are things a banned player can do, like coming to the forum so we can explain the rules. When they say sorry on the forum (and I believe them) I will many times just unban them right away.
What you want to keep in mind is that we want players to understand and respect our rules, for ourselves but also other players. Banning is not the goal itself, it is just a way to get their attention and to "motivate" them to stick to our rules on our servers.
In the end you just use your judgement about the player breaking our rules. Did he do it because he didn't know the rules, or maybe had a bad day or did he do it because he just wants to ruin the game for others and thinks the rules don't apply to him? Both can only just have glitched but the first would get maybe a 3 day ban and the other 1 or 2 weeks.
It's an art