First, thank you for listening to the community feedback and replacing those articles with themes that we can debate and share our experience about.
The ideal length of the period will depend from aquarium to aquarium and is hard to find the exact duration that gives maximum returns with minimal adverse effects.
Also it is important to understand that light strength and light duration are not in a simple mathematical relationship; ie. keeping a light at 1/3 strength for 18 hours is not the same as keeping it at full strength for 6 hours.
That being said, I start new aquariums with 6h and then go up. All plants I had survived at this light duration. However I noticed red plants become redder as the light duration increases.
I also suggest 8h is a good spot. 12h would be the top for stronger light.
Some plants will close up when their photoperiod is close to the end (Althernathera, Rotala..). Run a stable photoperiod over a long time. Then start the photoperiod 4 hours later, you will see some plants still close up at the previous closing hour.
Here are another things to consider:
How long does it take for the plants to change their internal clock and adjust to new light schedules?
What is the impact of environmental light ?