These are the averages from my science experiment that I had to conduct.
Milk chocolate melts at approximately 3 min 36 sec at boiling point.
White chocolate melts at approximately 4 min at boiling point.
Dark chocolate melts at approximately 4 min 08 sec at boiling
High-fat chocolates (milk, semi-sweet, white, ect.) will melt the fastest because their melting points are just below that of human body temperature. Most rooms are kept at a stable temperature for people, so melting that type of chocolate does not require too much extra heat. Also, this is why those chocolates have the tendency to "melt in your mouth", or melt in your hand as heat is radiated and trasnfered from the skin to the chocolate.