I'm pretty sure you must be on the defensive to use force...i.e. the guy must be grabbing for a weapon, ready to use it on you, or already use it on you before you can do something.
You are allowed to use reasonable force. So if the guy has a knife and you grab your Louisville Slugger and hit him a couple of times, that's OK. However, once he's down, you have to restrain from attacking any further. Basically, in any sort of physical fight, you have the right to defend yourself until you are capable of running away to a safe place.
I remember in grade 9, cops came to speak to our school about violence, and they told us that if we were ever attacked in a fight, we could fight back until an opportunity arose where we could safely get away. That translates practically, into get the other guy down, but don't hit him when he's down.