Never.
Even the self defense argument is debatable, considering how much larger and stronger men are than women. You are very likely to be able to subdue her without hurting her.
For example, most women I've dated are about 100-130 lbs. They possess what I would guess is 1/3 of my strength, therefore trading punches is not going to end well for her even if I show an immense amount of restraint. And in all likelihood, the LEOs are going to take her side on this one.
If you have to defend yourself or feel the urge to hit a woman, it's best you just leave. And don't forget to document it... My ex-wife hit me on a few different occasions. It was my "ace in the hole" if she continued to push for full custody, even though my attorney felt I should divulge it initially...
If she's threatening my life or someone else's, all bets are off...