Elaine writes:
The activity continues for about 2 weeks from my experience. I know that because when I used to use some of the previous batch for a new batch of yoghurt, if I kept it longer, my yoghurt did not turn out right. Considering that bacteria require only a smidgen of nutrition to keep them alive and multiplying, I would think there is, first, a bit of residual lactose, then there is galactose which, as long as they stay alive, they may adapt to using.

I have books on yoghurt but I never came across that question. Most of us are satisfied to know that we can enjoy a good source of protein (casein) and calcium and make delicious foods with it as well as delivering some good guy Lactobacillus bulgaricus to a teeming population of unfriendly bacteria - the more good guys that get down there, even if their residence is transitory, the more they knock the bad guys out of the territory and into the toilet.

We also know that the yoghurt bacteria produce lactic acid from the breakdown of lactose into glucose (plus galactose). It might be possible that they can use some of the lactic acid.

I would also add that from my experience in trying to figure out what is happening with probiotics, a lot of what is published is merely information from in vitro (petri dish or test tube) experiments.