I have a 8 months old Bullmastiff puppy,when do I start to give him adult dog food? Thanks

I don't know what type of food you are feeding your dog, but if you put him on holistic or natural foods, then there is no transition to adult food. If you are feeding him regular retail dog food, then you could probably start any time.

I would strongly urge you to lean more toward holistic and natural foods for your dog, as they are much healthier, and contain far less fillers and bi-products. Because holisitc and natural foods have less garbage in them, your dog will end up eating less, therefore pooping less because their bodies absorb more of the essential nutrients from the food. You'll find that the coat and teeth are much healthier too.
I agree with JustaBrat. But if you nonetheless decide to stick to retail dog food, the right time to start giving adult food depends on the size of the dog. Basically, toy and small breeds can be given adult food at about 5 months, especially if they are neutered/spayed. Medium size breeds such as Cocker Spaniels can be fed adult food at about 8 months. For large breeds (Golden Retrievers for example) this age is about 12 months. Giant breeds such as the Great Danes must stay on puppy food for about 18-24 months.