How long does it take for Drupal to unblock user after 5 failed attempts?
To answer your question:


  The amount of failed logins is recorded in the table 'flood'. You can
  either wait before trying to login again (6 hours) or clean the
  flood table with the procedure below.


Options for not having to wait:


  Execute the following query on the Drupal database:
  
  DELETE FROM 'flood'; If command above doesn't work try this:
  
  TRUNCATE flood RESTART IDENTITY; To execute this query it will be
  necessary to login to the database. This is typically done through the
  command line or through a GUI interface such as phpMyAdmin. If Drush
  is installed on your server, the drush sql-cli command provides quick
  access to an SQL command-line interface.
  
  From the command line, with Drush installed:
  
  drush php-eval 'db_query("DELETE FROM `flood`");' NOTE: The query
  above will delete ALL entries in the flood table. If you want to
  remove only a specific user's entries from the flood table, the
  following is more specific.
  
  DELETE FROM flood WHERE event = 'failed_login_attempt_user' AND
  identifier LIKE '1234-%'; (where 1234 is the UID of the blocked user)


Source: Login blocked after 5 failed login attempts

Want to change the settings for this? You can by installing Flood control