How long does automatic “New user typed too fast” silence last?

No, there is not.

The most basic flow is like this:

  • system silences a user (for whatever reason)
  • it appears in the Review Queue
  • moderators deal with it

There is no PM, no other notification indicating a user was silenced for typing too fast etc.

The only reason I know there are these random times without notifications is because I regularly check the Admin >> Users >> Silenced tab.