I've never cooked a stock covered. Not even a little bit. My stocks reduce, and I add water again. I'll do that a few times throughout the cooking cycle which for me is at least 20 hours, sometimes close to 24.
I complained a few days ago, that my current stock was too strong in flavour. It was made with an old stewing hen and simmered 22 hours or so. Anyway, when I made a soup, I used 1 part stock, to 1 part water, and added 1/2 boullion cube. It tastes nice.