Within this work, we cannot provide an exact borderline on how long an authentication mechanism for ATMs should
be. However, we argue that PIN authentication is only accepted by users since it is very easy and { maybe most importantly { extremely fast. Therefore, it is highly appropriate for ATMs, since the overall task is very short and PIN still
only requires a small fraction of the overall time. A rule of thumb could be that an alternative authentication mechanism for ATMs should only require a fraction of the overall time (< 10%) that a user spends at the machine.