MILK Chocolate or Sweet chocolate isnt that deadly. It can make them sick, but they can eat alot of it and not die. I bet a lab could eat a candy bar.
Now - UNSWEETENED chocolate, like bakers chocolate, is MUCH more deadly and a bites worth can be deadly for small dogs.
ETA:
From Google.
theobromine is what is toxic to dogs.
The good news is that it takes, on average, a fairly large amount of theobromine 100-150 mg/kg to cause a toxic reaction. Although there are variables to consider like the individual sensitivity, animal size and chocolate concentration.
On average,
Milk chocolate contains 44 mg of theobromine per oz.
Semisweet chocolate contains 150mg/oz.
Baker's chocolate 390mg/oz.
Using a dose of 100 mg/kg as the toxic dose it comes out roughly as:
1 ounce per 1 pound of body weight for Milk chocolate
1 ounce per 3 pounds of body weight for Semisweet chocolate
1 ounce per 9 pounds of body weight for Baker's chocolate.