The longest service that comes to my mind is the Mondays only
IC2013 from Leipzig Hbf to Oberstdorf in the Allgaü, which also features 2x218 diesel hydraulic traction for the final section! It takes just under 14 hours, and given the route it takes it's hard to imagine anyone taking it for the whole journey.
The summer only, and soon to be history extension of the
CNL471 Berlin-Zurich sleeper service to start from the Baltic seaside resort of Ostseebad Binz also takes just under 14 before crossing the German-Swiss border near Basel.
There are also some other weekend-only trains that make a 12 or so hour journey across Germany before ending up in Binz.
There might be other mega-routes which are even longer in germany, but those are the two that spring to mind as real oddities. They each have one end of their route on a single track branchline which normally only sees 2 car multiple units.