This is like starting to carry a gun after being the victim of violent crime. That kind of thing is a relatively low probability event. It's unlikely to happen to most people in their life--less likely to happen more than once, in general. People need to start carrying before an incident, not after.
If you want to have a gun when you need it, you should start carrying immediately and you should carry any time you possibly can. That's your best chance for being armed when it matters. Waiting until after an incident occurs to start doesn't make sense.
Also, as far as I can tell, most people, even those with carry permits, do not carry most of the time. I'd like to revisit the 256K people who applied for permits in a couple of years and ask them some questions to find out how many of them are actually carrying, how many of them got any training beyond the minimum required to apply for a license, how often they are carrying, how often they are doing some kind of personal practicing with their firearm, etc.