Of course, many people get “arrested for defending themselves” because in general people suck at the “stopping when he’s no longer a threat” part. Carrying on hitting him because “he might get up again” doesn’t count, and kicking him is even worse – boots count as lethal force in many jurisdictions.
Eh. This is one of those things that you have to judge very carefully, and situationally. If you have every reason to believe, “If this guy gets up again, he’s going to continue to attack me,” then it’s absolutely reasonable to say that the point where you’re no longer in immediate danger is the point where he is no longer able to get back up.
And you also have to be very careful when it comes to considering exactly how much responsibility a person has to stop and carefully analyze exactly how much force they need to use to not be killed or hospitalized in the next few seconds. Is it reasonable, in this or that specific incident, for a person to assume that the attacker will make another attempt after they get back up? How often is it remotely reasonable to give someone who just attacked you that benefit of the doubt? Hell, how much is the average civilian expected to be able to measure in a desperate state exactly how much force is necessary to protect themselves and exactly how much force will injure their attacker how severely?