That's what I meant - the prerequsite settings in the lesson module settings do not grey out the subsequent lessons - because they are not part of the "conditional activities" settings that Do do the greying out and hiding; they are two differently designed entities, hence why I don't think you can do it. (Unless I have misunderstood you) You can grey out subsequent lessons based on the first lesson being viewed or having a grade, but not on a certain amount of time being spent on it.
Lesson
unavailable lessons greyed out
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