When issuing suspensions on a user, the recommended lengths are:
1st suspension: 7 days
2nd suspension: 30 days
3rd suspension: 365 days
However, it was originally:
1st suspension: 7 days
2nd suspension: 30 days
3rd suspension: 120 days or deletion
Why was it originally 7/30/120-or-deletion, but later changed to 7/30/365?
There's also another reason why accounts are not deleted purely for behavioral reasons if they have contributed any useful posts.
As of 2012 (possibly late 2011?), after the original policy of deleting accounts in such cases was created, the system was changed so that when an account is deleted, their username would be anonymized to their user ID. (Previously, it would just gray out their username without anonymizing it; you may have stumbled across a few old non-migrated posts from grayed out authors.)
This, however, could be considered removing the attribution, which would be a violation of the CC BY-SA license (all versions that SE has used), unless the contributor specifically requested removing it. So, to remain on the safe side, SE changed their policy to not delete accounts unless the user voluntarily requests it or the account has no visible posts. (SE makes it clear to users who request account deletion that they're also requesting under the license to have their attribution removed from all posts.)
(There are two schools of thought regarding user anonymization upon deletion actually removing attribution: some believe that your attribution is being removed since your name is no longer on the post, while others believe that it is not being removed since your user code is still attached to the post, which still uniquely identifies you later on. But as the answer to the latter post says, SE went by the former interpretation.)
On How long does a ban / suspension on an SE site lasts?, Grace Note says:
"7/30/365 is the progression as prescribed by the interface itself for dealing with a user who is a repeat offender, which thus basically means it is what we suggest for moderators ourselves. Originally it was 7/30/120-or-deletion, but since we're no longer really in the punitive deletion methodology, we switched to 365 days."
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