Hello,
Without knowing more about the health status of your dog and the size and type of chocolate eaten I cannot answer this completely. The pet poison helpline can calculate the answer based on a series of answers to questions they will ask. This is the best place to find the answer to this question.
In general a healthy normal pup with a small milk chococolate is unlikely to cause an issue.
There are also several “choc tox” apps available for free online. They may help provide some guidance.