The only reason to do this is if your light struggles to provide enough DLI. (daily light integral - read wiki for basic understanding)
if you provide enough DLI over 16h, that's fine. OVer 18h? that's fine. The plant absolutely benefits from some length of darkness and absolutely does not benefit when you give it more DLI than it can handle. (this amount varies due to local variables - Temperature, Relative Humidity, and atmospheric CO2 levels) -- You must derive the 'max' through trial and error.
Doesn't matter if you give that same DLI over 12h or 18h. Length of time is relevant but on its own is useless. Rate x Time = DLI. Number of photons absorbed per day is what matters.
Adjust light intensity or hours of operation (both can have same effect) based on plant growth pattern. If nodes are too tight, reduce DLI. If nodes are too stretched out/lanky, then you need more DLI.
The area you try to cover with the light will obviously be proportional to DLI too. Try to cover twice the area, all other factors remaining the same, then you have half the DLI across that larger area.
hours and umol/s PAR are directly 1:1 proportional and area (relative to 1m^2) is inversely 1:1 proportional to DLI. You can infer this from the math explained in the wiki for DLI.