Hmm, Certina's FAQ page states:
14. How much is the power reserve for a self-winding mechanical Certina watch and how does it work?
The functioning of a self-winding mechanical Certina movement depends on the wrist movements of the wearer. When fully wound, the watch have a power reserve of minimum 38 - 42 hours. Since the watch's self-winding mechanism is directly linked to the wearer's activity, 10 to 12 hours' wear should be sufficient to generate a power reserve of 20 hours or more, which ensures that the watch will continue to run throughout the night. However, if a watch has not been worn for several days and its power reserve has thus run low, we recommend you wind the watch manually for around 15 rotations of the crown (clockwise) to ensure the best possible rate from the outset.