Discus really don't care about light.
they actually inhabit areas that don't get much sunlight
I keep the lights off untill I get home then turn them on for about 4 hours.
they do fine with low light or ambient light from the room. they can find the food fine in low light.
it's more for your plants than the fish.
True, the plants and the viewer! I have mine on for me, not the fish. It is about 1.5 wpg on my tank so it isn't too bright for them.