How much exposure?

For the summer of 2015, my three year old son will be taking some private French lessons with a charming young girl. His teacher is only about fifteen years old, but she is a native French speaker and very enthusiastic about teaching children French. 

This leads me to the question, how much time should a toddler spend on foreign languages? Should there be a certain daily allotment to foreign language learning? The amount of daily exposure depends on your goal. My goal is simple, I want my son to naturally absorb syntax (natural arrangement of words) and phonological awareness (basic sound units). A systematic exposure to language is more important than huge amounts of time surrounded by the language because the toddler  brain works on QUALITY more than quantity. Therefor, regular systematic exposure with stable speakers will help a child reach my goals.