In Germany, you are only allowed to drive a motor vehicle with a foreign driving licence for six months. It must be rewritten within this period, otherwise the driving licence becomes invalid and driving is no longer permitted.
The period of six months starts from the day on which one has a "normal residence" in Germany. For a "normal residence", you have to live in Germany for at least 185 days a year.
If you have not been ordinarily resident in Germany for more than 12 months, you can extend the six-month period by a further six months, i.e. to a maximum of one year.
Within the six-month period, you must carry a translation of your driving licence if it is not in English.