How much is too much?
Am I swimming? It's easy to have too much when I am swimming.
As an absolute, I don't think you can come up with a rule. Maybe when your ammo stockpile exceeds the value of the structure you store it in, it might be time to reprioritize expenditures.
Honestly, as long as you can safely store it, there is no such thing as too much. Financially it's much easier to have too much.
It's a lot easier to define too little I think. IMO if you have less than 250 rounds for a caliber you shoot, you are running low. My reasoning for that is that given an hour of range time, I can run through about 100-150 rounds without rushing (at your average indoor by the hour range anyway). If I only have 250 and go to the range, I'm basically one range trip form totally being out of ammo byt the time I get home.
Basically, I usually buy 1000 rounds for each handgun caliber, 500-1000 rounds of most rifle calibers, and 250 rounds or less of anything you can tell you went shooting the next day after less than 25 rounds. When I hit about the 25% mark on inventory, I feel the urge to top up. Whatever handgun caliber I shoot in competition, I buy reloading supplies a years worth at a time, and get the urge to restock when I hit the 6 month mark.
That's what I consider enough, but if I could sock away 10 times that and still pay all the bills, I wouldn't hesitate.