If you start and not drive, you should leave it running for at least 30 minutes or more. Short time startups only "kill" the battery faster. Also, water will stuck in your exhaust pipes/mufflers which will rust them out fast.
Twice a week is a lot more frequent than necessary. And if you just start it without driving, you are doing more harm than good. Once every two months is OK if it stands in a place where the temperature and humidity is stable. But then you have to take it out for a drive long enough for the car to reach full operating temperature, I would say at least an hours drive. Remember to keep your gas tank full.
If the temp and humidity is unstable and unfavourable, once a month. Again, at least an hour.
Disconnect the negative battery terminal in the mean time. Recharge the battery before storage, especially if it's cold. If you didn't change filter and oil just prior to the storage, do it now. If the storage lasts for more than a year, change oil once a year at least. This is due to chemical prosses, not pollution and wear.
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!