As was stated by tom tulpe, DM banknotes and coins dating from 1948 and later can be exchanged at any time by the Bundesbank. Since you say you aren't going to be in Germany any time soon, the page tom tulpe linked to contains a form (available in English) you can send in to the Bundesbank office in Mainz along with your banknotes and/or coins; they will either transfer the money or send you a check.