When is the best time to start flushing in 30 g soil smart pots?

I know I should have marked the date on when they started flowering but I didnt so is there a way to read the trichomes or something to estimate when to start flushing?

it kind of depends on where you are based on light and season .... I'm growing 30 gals in a greenhouse and I'm about 2-4 weeks from done. Northern CA Bay area... I went to every 3rd feeding (water, water, feed) about a week ago. I'm probably going only water in about a week