Porsche recommends treating German cabrio tops once a year or whenever the top stops beading water.
How-To Detail Convertible Tops - http://www.autopia-carcare.com/inf-cabrio.html
RaggTopp is highly recommended for waterproofing the canvas convertible top. It is recommended to use this product at least twice per year, and/or when water no longer beads on the top fabric. Overspray is not a concern with this product, as it is with other top protectants such as the OEM Porsche top treatment. It appears that the only place to purchase Raggtopp may be online.
Porsche Tequipment Soft Top Wash Shampoo and Cabrio Top Protectant are the company's own solution for top protection, and is available at most dealers plus some online stores. This product requires critical masking work to keep the protectant off painted areas.
BMW Convertable roof cleaner and Impregnating agent present another alternative provided by the Bavarian competitor. It is reportedly very easy to use. No masking or spraying required. Just sponge it on.