Should you flush weed and how much in advance?

Probably looking at 1-2 weeks before harvest .it has begun to fade but i think there will still be plenty of nutes in the leaves by the time the bud is ready ,

wondering is this a sign i should flush and let the plant cannabalise over the next two weeks feeding only water after i flush with 10 20 litres of water twice a week ?

what about 48 hours of darkness before harvesting ive read this is beneficial for trichomes ?

No, Cut down the nutes 2 weeks before you plan to harvest, if using chemical fertilizer. I cut 25% the second to last week before harvest week, and then another 25% the week before harvest.
Flushing is a myth.

If your plant is already senescing/fading, then please understand that's really all that flushing is designed to do on a very basic level, assist the plants with fade by forcing the plant to use its contained nutrients and stop taking up any. . .. Your plant is already doing that naturally, don't mess that up.

Also highly agree with @jimihendrix1 as it pertains to this topic.

48 hrs darkness is also a myth, BUT I'll admit the general idea of it potentially boosting trichomes or potency makes sense via stress response. . . it's just that, trichomes take weeks to develop and more weeks to ripen, WTF is 48 hours over the course of weeks? --nothin.

Just keep the plants happy and healthy til chop, is your best bet.