Is hard to say how many grams, I would say 30/40 gr per plant.
regarding the harvest, you must check the trichomes with a magnifier, If you’re harvesting with a majority of milky or cloudy trichomes present, you’re going to have a very THC-heavy high, which means effects such as memory loss, giggles and more psychoactive effects of the plant will be at their strongest.
For the best results, you want a nice healthy balance of milky trichomes, with a few dark and maybe a few clear just to balance things out. This will produce the most well-rounded high where you’re getting a nice share of all the properties marijuana has to offer.
However, you could simply harvest the plants when they’re at the most potent level of THC if for example, you’re taking a CBD oil supplement. Then, when consuming your cannabis you dose alongside it with your CBD to balance out the high.