Never,
the first time my ex was physically violent directly to me, he went for my throat, it was when I tried to leave him, after spending 3 years enduring emotional, sexual abuse, physical intimidation, and generally violent behaviour,
I once asked his friend why no one had told me, warned me what he was really like, and he said he was sorry but they, as in his friends, had thought he had changed
So no, they cant change, they can try, they can lie to themselves and their friends, but they will never change, violence will always come to the surface,