First, it might be helpful to define the word flush. You ask how long to do it and in my world a flush will take about 15 minutes. Let's make sure we understand that we are talking about the same thing.
Best time is never, but that's only if you're growing 100% organic in living soil. Because theres nothing to flush.
If you must flush salt from your soil the best is when you notice salt buildup and or just every few weeks no matter what to keep the salt from building up.
Flush with 3x the container size of water mixed with Yucca extract to help wash the salt out.
But dont just water your bottle fed plant the whole final two weeks, that serves no real purpose.
You'll probably lose 10% yield.
Hey @Quality G, I forgot to welcome you to the forum! You asked a great question and one we have all been debating here for a bit. Expect a lively conversation to result!