There is only one answer to this question: as soon as you experience the abuse.
There are essentially two options. The first is to have recourse to the civil justice system, or in other words, to bring a court proceeding to seek a restraining order. The second is to have recourse to the criminal justice system, or in other words, to call the police. In my view, the better course of action is to immediately report any incident to the police, for the following reasons:
It's worth pointing out that statistically, a person reports abuse to the police only after he or she has been the victim of repeated assaults -- on average, more than 30 times. My advice is not to wait for the abuse to continue, as it likely will, and to take action the first time. Hopefully, it will also be the last time.
Herschel Fogelman practices family law with Basman Smith in Toronto. He is also a mediator and was a script consultant to the television show The Associates.