What is the correct consistency of a cream soup? Some cookbooks give advice, but it is conflicting. Some say "it should coat the back of a spoon", which I find terribly confusing - my best try to imagine the result will work with a wide range of consistencies. Others don't give instructions at all. I've had recipes say "then add milk to reach cream soup consistency", and I have no idea how much milk to add. 

Eating out doesn't make it easier. Our cafeteria offers stuff I would classify as vegetable puree at the soup bar. I'm sure I've had "cream of mushroom soup" somewhere which had the consistency of half-fat dairy cream. 

What is the correct, standard consistency some recipes refer to? Is it different for classes of soup (e.g. roux based soup vs. pureed vegetable based soup)? Or is it different for each type of soup, e.g. should a cream of mushroom soup be thinner than pumpkin soup? And how do I recognize that I have achieved the correct consistency?
The 'correct' consistency is very opinionable. The 'coat the back of a spoon' instruction means that you dip a spoon in the cream, then run your finger over the back of the spoon, and if the soup stays apart, it's at least as thick as it should be... but maybe thicker (than you'd like).

The problem is the amount of time that the soup stays on the stove, as it keeps on thickening as moisture evaporates. That is why you'll get heavy soups. Add water, to make the soup as thin as your own preference.