IMO a person who carries a firearm for self defense should be proficient with it. That person also should be proficient at other things that can help avoid using that firearm if at all possible to including:
-conflict resolution and avoidance (as previously posted)
-legal issues surrounding the use of a firearm (as stated previously)
-physical fitness (running specifically might be important)
-hand to hand combat
I've been reading "In the Gravest Extreme". I really like a lot of what Ayoob says -- The idea of carrying a 20 spot (accounting for inflation) wadded around a matchbook to handover to an aggressive thug is a good one. I'll pay 20 dollars every time to avoid something that could escalate. Now that I carry a gun, I'm much less likely to actually need it than I ever was before. Funny how that happens.
How much training is enough? For me it's as much as I can reasonably get, and I have a ways to go.