Allot of different opinions on this subject.
I recommend to wait until 18 months if possible.
no sooner than 12 months.
In large and giant breed dogs I do believe to wait until they have done most or all of their growing and are of a mature age.
I do believe that hormones play a big role in the development of a Great Dane.
I also believe a dog should be of age to handle anesthesia.
Always remember that what is good for a small breed is not always good for a Giant Breed.
article link below.
Please do research of what can happen if you spay or neuter your Great Dane too early.
Done right at the right age it is the best way to go.