I've heard many times not to expect them to get it and be fully house trained until 1 year. Before that you should know that there will be accidents.
We trained our dobes to use a bell... to start training, everytime we let them out we ring the bell.... keep the bell on the door you want them to use and low enough that they can either reach it with their nose or paw.
Then you teach them to touch the bell with their nose and everytime they touch it the door should open for them to go outside... it doesn't take long for them to put two and two toghether.