I’ve often wondered the same thing and thought of it more as a question of “Is there a point where light intensity, e.g.; identifiable PAR/PUR values, and photoperiod combine to damage plants?” I don’t believe that I’ve reached such a point, with my setup, if one exists. So, I will describe what my plants are exposed to, regarding light.
First, CO2 is unlimited; I run it 24/7 and target an ~30ppm level. I also have never detected macro (includes Mg and Ca) limitations. We can’t effectively measure micro levels, so I can’t affirm my belief that they are not limited.
I have two photoperiods, separated by a 3-4 hour siesta. I say “3-4” because each photoperiod has a half-hour ramp to/from zero artificial light (ambient room light can allow plants to photosynthesize). I am not implying that a siesta is beneficial. I think it is, but haven’t proven, to myself, that it is.
The first photoperiod has about 40 PAR at the substrate (14” of water) for two hours (viewing purposes only), again surrounded by half-hour ramping. The second photoperiod has about 120 PAR at the substrate for seven hours (same ramping). PUR values are very high. I haven’t gone beyond these variables only because there is no point: no one is in the room enough to enjoy it, outside of those hours.
With the above settings, I have seen no detrimental effects with my plants. However, my plants are all stems, with the exception of some Dwarf Sag and DHG (mini). I have noticed that some stems (my Limnophila Aromatica, Pantanal and Wallichii) begin to close-up when the light begins to dim during the last photoperiod. This is something they learn to do. They also did it when my PAR values were much lower and when that second photoperiod was shorter, so it is triggered by a learned response to their end-of-the-day life, and is not photo exhaustion.
With these settings, I also have no algae. Some people would say that’s a lie, but I can’t see any in my tank (I had plenty in the past). I don’t attribute this entirely to plant health, although I think it is the majority of the reason. I believe that my snails do enough damage to the surface biofilm to do the rest of the algae inhibition.