If roller shutters are to be kept in great condition then they should be thoroughly cleaned every 12-18 months. They are exposed to the elements on a daily basis and the horizontal slats are absolute magnets for dirt and grime.
It is usually advisable to do so using a damp cloth as abrasive cleaning rags and harsh chemicals can damage the surface. Simply roll them up a small amount to expose the gaps between the slats and run the damp cloth across the surface, also ensuring any twigs and leaves are also removed. Then rub down the roller shutters on both the inside and outside with either water or a neutral cleaning agent, before rinsing off with more water and then drying immediately.