The answer to "how much chocolate is toxic" varies widely on the type of chocolate, the size of dog or cat, and how much they have consumed. Here are some guidelines and calculators to see how much is too much for each situation.
The toxic dose of Theobromine (and caffeine) for pets is 100-200mg/kg. (1 kilogram = 2.2 pounds). However, various reports by the ASPCA have noted problems at doses much lower than this - i.e. 20mg/kg.
Translated to a typical scenario, and using the 20mg/kg as a measure of "problems can be seen at this level of ingestion", a 50-pound dog would have to consume 9 ounces (+/-) of milk chocolate to consume the 20mg/kg amount of Theobromine. Some dogs won't see problems at this rate, but some may.
This is a much more conservative toxic level calculation than the standard of 100-200mg/kg, but better safe than sorry. The bottom line: A dog sneaking a couple M&M's shouldn't have a problem, but it isn't a good habit to get into.
Chocolate Toxicity Calculators
Please note: This article has been provided for informational purposes only. If your pet is showing any signs of illness, please consult a veterinarian as quickly as possible.