It may vary depending on illumination intensity of light the solar panel of the watch receives from the light source.
If an Eco-Drive watch with the solar panel under the dial is placed flat, it takes longer for charging in winter than in summer. Since the altitude of the Sun in winter is lower than that in summer and consequently illumination intensity of its light is weaker in winter.
In addition, the watch is often covered with a sleeve of the coat or outer clothes and may not be exposed to light enough in winter. It may cause insufficient charging.
It is recommended to expose the watch to light facing the dial (solar panel) vertically to the light source in any season.