"Too much"... to me that implies, "the point at which disadvantages start to appear, particularly those of significance."
By that definition, "too much" would be any useful caliber/cartridge that you can't shoot as well as your best shooting cartridge. If you shoot .380 and 9mm equally well, but find that 40 and 45 slow you down a bit, then those are too much and 9mm is the best. If you shoot .380 the best, then 9mm is too much. If you're a manly man with uber skills and shoot everything up to 357mag the same but 41mag slows you down, then 41mag is too much.
I'm sort of in a tossup between 380 and 9mm being just right. Anything more than 9mm and I can't shoot it as accurately at speed.... at least, not with any gun I've had the opportunity to work with.
EDIT: This is assuming 2-legged predators. For animals, it's a different game. Follow-ups are important, but power becomes more important. I'd say for most people anything bigger than a 44mag is too much.