Usually it takes anywhere from 60-90 minutes. Even larger chickens tend to be done in the same amount of time.
First time I spatchcocked a chicken I fell in love! It's the only way to cook a whole chicken in my opinion. Best part is I show my friends how its done when I'm doing one and they always end up doing their own later.
Oh try the yardbird rub too, its amazing!
PS. I've been lurking for far too long this is my first post!