I have loads of experience but I make no guesses as to how long I would last. All it takes it one goober with a lucky shot or some unforseen mishap to do you in. As has been said so often in military circles, and I paraphrase, you can plan all day long but when the first shot is fired the plans go in the trash most times and you are winging it.
Hope for the best and plan for the worst.
That is certainly my mantra. I've been involved in several active shooter situations. I've been shot at and been the shooter. Nothing prepares you for that. Nothing. I don't care how much or how little training you have, when you start pulling that trigger, you're relying on your training to automate your response, but psychologically you are never ready.
“The first casualty of battle is the plan” is the version I am most familiar with; I use it at least once a week. It is a good thing, train your military leaders (or yourself, in this context) to expect the unexpected, and to become comfortable operating at high optempo in an environment with high uncertainty.