The heat in bitches can vary a lot, depending on the size, breed or even the individual physiology of each dog. The normal period for the first heat is 6 to 9 months of life. Often, the first heat has fewer signs such as bleeding, and changes in behavior, being called a silent heat.
As for the ideal age to be able to breed the dog, it will also vary, the recommended is always at least in the second heat, but large dogs like the Great Dane may not yet be fully developed in the second heat and, thus, not having a good pregnancy .
For a better explanation in relation to your dog, the indicated is a consultation with the veterinarian and, with a clinical examination, he will be able to check the development of your dog and provide guidance on the possibility of breeding.