My Great Dane Sadie was about nine months old when she had her first heat. I would expect yours to go in heat this spring. Mine didnt have any trouble with her heats although I found she produced alot more blood than my collie mix when she would go in heat, so you might want to look around for some doggy diapers for her in advance!
I know that Sadie would leave a big mess if she was lying down or if she got excited and wiggled ,it would kind of fly everywere. Hopefully for your sake she will keep herself clean, Sadie wasnt very clean about it, but my collie was very clean. You could hardly tell when she was in heat. It was quite obvious when Sadie was in heat though! I hope this gives you an idea of what to expect at least. Good luck and welcome to BPO. We would love to see some pics of your girl if you have some. We all love pictures.