As mentioned above, baker's chocolate is much worse than milk chocolate, since it has much higher levels of theobromine. A 1 oz piece of baker's chocolate can make a 40 lb dog sick, but it would take around a pound of milk chocolate to cause the same problem in the same dog. And it takes even more to become life threatening. Basically, if it's milk chocolate, unless your dog got into your large stash, he'll be fine.
In other trivia, chocolate is even more toxic to cats, but cats are unable to taste sweet, so are generally uninterested in eating it.