i'm not sure there's an exact grains-per-quart ratio because i think since the grains are living some are more productive than others, i think it just requires some experimentation to find the happy ratio that works best for you (that took me a while, and changes if i let them hibernate in the fridge for a while).
i do use raw milk and IME the look and taste and smell of milk that's turned is totally different than that of kefir. i think it's going to depend on how fresh the milk is and what the ambient temp in the room is as to how long it would take the milk to turn. but i've never had a problem with it doing kefir (meaning, never felt that the milk had turned before it had sufficiently turned to kefir). i suppose you could also do that as an experiment, leave some raw milk on the counter to see how long it takes to turn. though of course it will still be longer if it's in kefir grains because the grains will start working on the milk right away. and i would think giving it a good stir/tipping the jar regularly would help prevent that too, to make sure there isn't milk at the top that hasn't gotten near enough to the grains to start the process.
of course i keep chickens, so if i ever leave it out too long and find it slightly questionable, i just give it to the chickens

they've never had any trouble with it, and actually love the stuff and seem to lay better when they get it regularly. anyway, this leads me to believe that even if i might find it questionable it's probably just fine to ingest, though it might have a stronger flavor than you prefer.