It seems to be incredibly varied throughout the breed based on genetics, diet and nutrition, exercise, and I'm sure some other factors I'm forgetting. My female just turned 17months is 83lbs, she has had obvious growth spurts. She would just look tall and gangly all the sudden, then fill in with mass but her head would look tiny, then the head would catch up, this process seemed to happen about 3 times. My male is 10 1/2 months 92lbs his growth has seemed much more steady, sometimes I swear he's growing right in front of me. The rule you're referring to is a very loose general one and by no means an accurate representation of most corsos. The seemingly more accurate rules I have found thus far for growth are that bone and structure growth occur up to 12-17months old then mass is gained until 24-36 months old. Just remember trying to force a dog to grow to a bigger size or speed up the process not only is a near impossible task but also a dangerous one with one of the best outcomes being an overweight dog with sketchy joints