In this answer to a previous question of mine it was mentioned that the fermentation phase of yogurt making has 3 phases of growth - lag, logistic and plateau. This means that the growth rate follows approximatly logistic function or S-curve. My understanding is that steepening left of the curve represents maximum bacterial growth rate unconstrained by availability of lactose, and the flattening right of the curve is the lactose running out as it has all been converted to lactic acid.
Online instructions for making yogurt frequently suggest different fermentation times for different qualities, for example the top hit on google says "6-8 hours produces a mild yogurt, 8-12 hours a tart yogurt, and more than 12 hours produces a sour yogurt."
Roughly where on the S curve would these points be? Would they all be in the growth phase, such that one would expect a 12 hour yogurt to have lost far more lactose than a 6 hour yogurt but still have substantial amounts of lactose remaining? Would they all be well into the resource depletion phase, such that most of the lactose would have been used by 6 hours and by 12 hours almost none is left?