What are the correct simmering times for chicken/vegetable stock?
It is true that flavour is affected by cooking time. Most likely nutrients as well, but its about the vegetables. If you cook beef stock you should indeed cook it for several hours to release the collagen and proteins. My experience with chicken takes less time so 30 minutes in the pressure cooker or 1,5 hours in a normal pot. Then you can eat the meat as well. 
Vegetables should only cook for 30 minuts so add the vegetables 30 minutes before the end of cooking regardless of the kind of soup. The fresh flavor of the vegetables will disapear if you cook it for longer then that. 
Someone doing research on the flavour development and time for making stock