Think about it. In the heat of shooting that may last seconds they wouldn't have time to think that way anyway.
In all the teaching I ever did, it was “if necessary, and the Officer life is in danger... ‘shoot to kill’ in this type of robbery and physical attack trying to take his gun. The guy is already a robber, and if he gets the gun, most likely he will shoot the Officer.
You are right, a few seconds at best... to make a life or death decision.