when will my dogue de bordeaux have her first season. She has just gone 8 months

At any time! Be aware of her "bits" - when they start to swell a little she is beginning to work up to it. When she is swollen check her each morning by pressing a white tissue against her vulva and look for pink staining. Then you know she has really started. Or you may come downstairs and see spots of blood on the floor.

Hope this helps. :)