I would say it would be about 150km from frankfurt to koblenz, then 400km to trier, then 150 along the saar.
you would follow the Main from frankfurt, then up the rheine, up the right hand side for about 50km, then cross over on a ferry at binghem an Rhine to continue up the left bank to koblenz, the mossel and saar routes swap over at various points to take you thhrough the best towns and villages, they are major cycle routes and can be downloaded of the net, I have gpx tracks for most of it.
Good thing about Germany is its pretty easy to jump on a train with the bikes to get back to frankfurt from pretty much anywhere you make it to, so no point worrying about making mileages.
http://www.romantic-germany.info/Moselle-Cycle-Route.4190.0.html