The invention relates to an electric tooth cleaning device and to a method for automatically indicating the cleaning duration when cleaning teeth, whereby a timer signal is emitted using a time element after the termination of a set cleaning duration in a tooth cleaning session. According to the invention, the set cleaning duration, whose termination prompts the emission of a timer signal, is modified. An actual cleaning duration in the tooth cleaning session is registered by a time registration device. The deviation of the actual cleaning duration from a predetermined standard cleaning duration is determined by means of an evaluation device. The set cleaning duration of a subsequent tooth cleaning session is then defined depending on the deviation that has been determined.