Whatever reasonable force is deemed necessary. Of course, everyone has a different definition of reasonable force but whacking with a rolling pin is maybe okay........
A policewoman I know told me that if, for instance, I was chopping onions in the kitchen and some one knocked on the door and I went to answer it with the knife in my hand and then that person forced their way in and I stabbed them, then that's fine as they would check the wound for onion presence. Of course, it's quite hard to make sure you are cooking at suitable times (when some one breaks in).