How awful for you! But I wonder...has the time come to consider professional care?
It really does sound as though your MIL is becoming extremely violent and is a risk to herself and others. It might well be that a short stay in hospital whilst ways to stabilise these periods of violence are sought, might be beneficial.
Reading between the lines, I have a feeling that you are hoping that the very difficult decision of placing MIL into care, possibly against her will, is something that you won't have to do. But as has been said, there are no hard and fast rules and this could go on for years, or it could just suddenly stop.
Have you sought any advice about this, from the GP, CPN, Social Services?
It really does sound as though you're not being able to cope, and you have to consider the health of yourself and husband. You can't go on like this.