How long does he have to press charges for domestic abuse?

My boyfriend's ex-wife refuses to move out of the house that he won in the divorce. Because of this, she continues to drop by unannounced on a frequent basis. She came over to his house last night to talk to him about something and things got heated and she ended up hitting him. He put his hands up to defend himself and apparently hit her in the face in the process. This is not the first time she has assaulted him. He has shown up to my house twice within three weeks with strangle marks and bruising around his neck. He has pictures for proof. Based off of last night's incident, she is threatening to turn him in for domestic abuse. What are his rights and can he file for domestic abuse charges? And if so, how long does he have to do it? I might also add that she has a history of assault. She was arrested two days ago for assaulting me.