I'm not sure you can do this - there are two things going on at the same time here, two differing sets of conditions. If you use Moodle's conditional activities then you can set lessons to be greyed out according to view, date, grade etc as you have observed. Alternatively you can use the lesson prerequsites to restrict access to the second lesson based on time in the first. The lesson prerequisites were around before the conditional activities and are a hangover from that time and isn't connected with them. However... let's see if others have an idea
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.