[essabee]

When you talk about plants, most people think that they react to variables in a set pattern - but that is not entirely true there are various degrees in that reaction even in the usual general direction and some exceptions to the general directions may also exist. Plant evolved in competition with other plants and survived by adaptation to various niches in the environment where they could out compete their rivals. So each plant specie is slightly different from the other species and even within the specie there might even be slighter differences between individuals. So optimal for one may not be so for another.

In natural habitats light varies in spectrum, duration, intensity not only over stretches of area but also with the time of the day as dukydaf pointed out above. The spectrum varies with time of the day - longer wave-lengths being higher in the early and later parts of the day. The spectrum of light of the stretches lighted by indirect light have lesser amounts of the reds. Intensities vary to the maximum when the sun is at the zenith etc etc

When it comes to your individual tank - you create a niche with with your light pattern, flow and nutrient regime. By varying the light alone you are changing just one part of the environmental niche; this presupposes the others were invariable - that's not true - so finding the optimal duration would not depend on the light itself.

Optimal for the hobbyist would be the sweet spot where all the variables are set in a way creating a tank environment which would cause the set of plants in the tank to grow well and out-compete the algae.