I have not had any batches of kefir going for a while, but I have had a jar in the back of the fridge for… well… since June I think. It is a dairy product, and it does not smell bad, even after 5 months. It does not exactly taste sweet, more sour and a tinge of bitter, but still drinkable. Yes, I did. I drank some. Over the months it had separated and the whey sat on top, which is what I think preserved it. Nothing grew inside (that was visible), and it still had that clean sour smell which is normal to kefir. Now, this batch did not actually start as normal kefir, but as a first or second batch after resurrecting the grains from hibernation. The stuff that Dom says to throw out. Not sure I will drink it all, but it was an interesting experiment into the longevity of kefir, and explains why cultures used kefir to preserve their dairy.