A shade less than two days (about 40ish hours give or take) is ideal. Check out this article: Yeast Culturing.
A good bet might be putting the yeast on the plate late-night on a Friday, then brewing in the morning/early afternoon on a Sunday.
Timing it to this degree of precision is impractical, in my experience. I've pitched both at high krausen and well after the ferment in the starter is finished with no perceptible differences between the two methods.
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