Originally Posted by
Flying Lawyer
As a US license holder, you are ok as long as you do not take residence. Your license is valid, however, you officially require a translation or an international driving permit.
Considering the case of a US salesman travelling from hotel to hotel you can be good for ever.
However, as soon as you take residence (residence is defined in sec. 7 StVG) you have half a year during which you can still use your US issued license (with a translation or international driving permit).
After this grace period you need to apply for a German license which can be quite painful even for some US citizens. Depending on the state your license comes from, you either need to take a full driving test, a theoretical drving test or no driving test. The most infamous examples are New York and California licenses, you need to pass a full driving test.
Just last week, I went to the Kreisamt in Bergisch Gladbach to get a new International Driver's License and asked the guy of the Führerscheinsstelle about this case.
US citizen, spending 95% of time in the US, US license (NJ) and also being registered in Germany (both citizenships) Basically close to no driving in Germany with either own or rental cars but once in a while it happens.
The clerk told me that my friend is safe with US/IDB as long as he can prove that he has not been to Germany longer than half a year. The reason is that he also needs a license in the US and therefore he can't just switch for short trips and have new one issued as he would have to give up "the other one" each time.