Around 25 miles...with car, when there is no traffic (haha) 25 Minutes...with normal traffic 1-1,5 hours. With public Transport 45 minutes.
BUT...the old Airport in Munich, now that one was a stone throw away, I walk in 10 minutes to the outer limits of the old one (thought its gone for 10 years now, time flies by).
By the way, we found a great thing at the Munich Airport. When we are there picking someone up, we normaly eat dinner or lunch there too, they have some great restaurants. They in return give you free parking tickets, cool deal, as the food is good and not more expensive than in other places.