discourse/app/jobs/scheduled/unsilence_users.rb

module Jobs
  class UnsilenceUsers < Jobs::Scheduled
    every 15.minutes

    def execute(args)
      User.where("silenced_till IS NOT NULL AND silenced_till < now()").find_each do |user|
        UserSilencer.unsilence(user, Discourse.system_user)
      end
    end
  end
end