As a fairly senior active duty sailor, when it comes to teaching a sentry/security patrol when to draw their sidearm and when to shoot, I try to make to it as simple as possible to understand:
• Unholster your sidearm when there is reasonable belief that you might need to employ the sidearm.
Note: if you have decided to fire your weapon, but must unholster it first in order to do so, it is probably too late.
• Disengage the safety of sidearm when you unholster your weapon.
Note: when carrying a sidearm while on watch/duty, it is Navy SOP to carry it in Condition 1 (magazine inserted, chamber loaded, safety engaged). When unholstered, the “brain & finger” now become the safety….which is why all sentries are constantly trained to keep their finger off of the trigger until they are actually ready to fire their weapon. I wont let any of my watch team carry a weapon (or stand the watch) if I am not confident in their ability to follow this rule.
• Fire your weapon at the threat when there is imminent danger to yourself or to others, but do not fire your weapon if you are not sure of what is behind the target. Reposition if possible. Continue to engage the target until it is no longer a threat to you or to others.
Yes, the decision to unholster can be especially tricky and difficult for the lawfully CCW civilian when faced with a developing situation. While unholstering a sidearm on military duty is somewhat different from unholstering a sidearm in the civilian world, the decision making process and principles remain the same. At least for me it does. Hopefully you will instinctively know what it is time to unholster your weapon and especially when it is time to engage the threat.
Obviously in the civilian world, an unholstered sidearm has all kinds of legal implications that need to be carefully considered BEFORE HAND. That responsibility should never be taken lightly, but you should be confident that you can make the right decision and are mentally prepared to make that decision as a situation arises.
As far as the speed of unholstering goes, keep this in mind: "slow is steady, steady is fast". My advice, practice the "when" thoroughly before the "how".