Is there a certain amount of chocolate that a dog has to eat for it to take effect?

There is not a certain amount. It usually has to do with what dog it is and how healthy it is. My pup ate a bag of reses pieces and they haven't been affected by it. I was lucky because my dogs were healthy and it was only half full. I'm pretty sure it has to be quiet an amount to kiil them but depending on which dog, it may harm them.