A method (300) for detecting excessive use of a data processing system, such as a computer or a video game, is disclosed. In a first embodiment of the invention, the method estimates (315 340, 365) the length of a period of continuous use of the system, typically a computer. When the estimated period reaches a pre-set threshold value, a warning is provided to the user (325, 345), in order to discourage he or she from looking at the monitor. In a different embodiment of the invention, the system (typically a video game) is disabled when its usage during the current day reaches a further pre-set threshold value. In this way, parents may limit the daily usage of video games by their children.