I have an unopened bottle of Helios brand kefir. It's dated 4/17/06 but doesn't say if that's a use by or sell by date. It seems like the kind of thing that would still be okay to use but since it smells kind of sour to begin with I don't know how to tell.
Kefir should just continue to get sour as more of the sugars are used up and it continues to ferment.
Saying that, if you have a bottle of store bought kefir from April (it's almost June) I would toss it. It's not worth risking there being other unpleasant bacterias growing, as the 'life' of the good ones might be at an end, from lack of food 'sugar' to keep them happy and healthy.
I'm guessing the worst is probably that it is REALLY sour tasting now and would be unpleasant to say the least...but on the conservative side, I would not risk drinking it, JIC. Store bought kefir strains are NOT as 'strong' and well keeping as Kefir made from Live Kefir Grains.