All, Generally (on average) and based on your experience, how much weight do Mastiffs gain between the age of 18 months and say 4 years? Bane is turning 18 months next week. He‘s close to 34in tall, and weighs about 215lbs. However, he still very much looks like a puppy. His face, he is pretty lean, and hasn‘t really filled out too much. A general idea would help. I understand that there are way too many factors to consider for an accurate prognosis. He was the biggest of his litter. Unfortunately, I don‘t know anything about his brothers and sisters. Some recent pictures below. Most people guess that he weighs between 150-170.