Maybe more important than stopping supplying nitrates is to give the plants 24 hours of darkness before harvest. Why? Because of starch. If you harvest at the end or even middle of the light period there will be a lot of starch stored up and that will result in more weight but also more harshness and lower potency, because obviously more inert weight will lower the overall THC percentage of that weight. Say there's 10% starch, potency is 10% lower. That's significant and I think plants store more than 10% starch though I don't have figures on it. Most starch would be used up for energy by the end of a 12 hour night but 24 hours makes sure it's all gone.
If you read about science class experiments about starch in leaves you'll see that they recommend 24 hours darkness to get a starch free leaf. That's my basis for suggesting 24 rather than 12 or more than 24. After 24 I assume a plant would start using up other compounds for energy. Might be good, might not. Nobody ever did real scientific experiments to find out. Maybe it starts using cannabinoids as an energy source eventually or maybe it uses up stuff that's not good anyway, like say proteins. Smoking protein isn't a great idea. All those legal Cannabis groweries in several states and nobody bothered to investigate the results of extended darkness and also nobody in Holland.