According to the terms these miles won't expire..."provided that you make a purchase for which miles are awarded at least once a month AND have held a Miles & More MasterCard for at least three months." (bolding mine)
Sounds pretty harsh as far as usual expiration rules but I'm assuming they mean if you don't satisfy those two conditions your miles will eventually expire at some time in the future, like 18 months or whatever just as most other programs? But I've never heard of that "once a month" activity requirement.