I would think that, after 48 hours and providing the temperature of the room was correct, the sugar should be fairly well absorbed by the bacteria.
For me, it seems the best temperature for fermenting kefir is from 74 – 76 degrees, but maybe others have found other temps to work fine as well. Anyway, if your room wasn’t warm enough for the full 48 hours, I suppose it’s possible that the kefir may not be fully fermented even after 48 hours.
What was the reaction you experienced?
I experienced a bit of mental grogginess/fogginess and a slight headache along with a body temperature rise. The room temperature is a bit hard to control, so I will try fermenting for 72 hours in case the kefir was not fully fermented. Right now, I feel like I’m making a lot of progress by eliminating sugars from my diet. If worse comes to worse, I’ll have to forgo the kefir route, because it will probably set me back. It tastes like grape juice!