Once you park take the road that's perpendicular to the main road until you reach a marking. You can choose to start with 5k now and 7k on the way back or the other way around. For some reason the route suggests you to start with the 5K. if you choose to follow the route counter clock wise and here you need to turn right.
Mini History and relevant information
Platteltz, a Romanesque keep, is the oldest part of the castle, having begun in the 9th century as a simple manor with an earthen palisade. In 1472 the Ruebenach house, built in the Late Gothic style, was completed. Remarkable are the Ruebenach Lower Hall, a living room, and the Ruebenach bedchamber with its opulently decorated walls.
Between 1490 and 1540, the Rodendorf house was constructed, also in Late Gothic style. It contains the vaulted "banner-room".
The Kempenich houses were finished about 1530. Every room of this part of the castle could be heated; in contrast, other castles might only have one or two heated rooms. (Wikipedia)
Anyway we reached the car at exactly 19:16 so 7h after we started our tour. I would say that hiking was about 4h. This makes out for a great day in the nature and a must see added to your hiking routes :)