Kefir should not taste like fresh milk. Kefir has a slightly sour/tangy flavor, similar to plain yogurt. When tasting Kefir here is what I do:
Stir with a clean spoon or shake the container to remix the separated Kefir. You just want to give it a simple mix.
Retrieve the spoon and taste it. Does it taste more like milk or more like a sour/tangy plain yogurt?
If you or someone else wants another taste, clean the spoon. No double dipping!
If it still tastes like milk I re-cover it and let it sit on the counter longer until it begins to separate again. Repeat at #1.
Below are a few stages of Kefir fermenting. All of these pictures are normal. Just stir the Kefir, taste it, and decide if you want it to sit longer on the counter. If you want to slow the fermentation process you can move the Kefir Grain jar to the refrigerator for a day or two. This will slow the fermentation process.
Fermentation Just Started
Separation of Milk indicates that the Kefir might be ready for drinking. Stir, taste, and decide: does it still tastes like milk? If yes, close back up and leave on the counter until it separates again. Stir, Taste, and repeat if the flavor still tastes like milk.
This is what I consider over fermentation. The whey has separated from the milk. The Kefir is still good to drink but it will be a little more sour.
After separating Kefir liquid from Kefir Grains (to start a new batch) you can mix your drinkable Kefir liquid in with a previous batch to blend the flavors. If you still dislike it you can feed it to your dog - it is good for them too. Google: "Kefir, canine, serving size, your-dog-breed" to find out how much to offer your dog.