How to know it´s time to flush?

From zooming on your diary images, you don't have any amber trichomes yet, most are milky but some are still transparent you have around 2 weeks left from these images. With regards to flush, you need to start the flush 14 to 10 days before harvest so maybe give them 1 to 2 more feeds and then start the flush. More details: Get yourself a jeweller's loupe (ideal, but a magnifying glass or even your phone camera will do) and check the bud trichomes. You will thank yourself because cutting early gives you the feeling of missed potential. Once you have looked at the magnified trichomes, you should look for the milky trichomes. The milky trichomes are full of THC. The transparent one's are undeveloped (less THC) and the amber ones are a bit overdeveloped because the THC will have started deteriorating. Since not all trichomes switch at the same time, I would aim for 20% amber trichomes. More amber trichomes creates more of a body high (usually associated with indica strain) while less amber trichomes gives you more of a head high (usually associated with sativa strains). P.S. Following pistils (the white hairs on the buds) alone is not reliable but you want to have most of them turn orange and start to curl. Good luck on the rest of your grow 😃. Looks like a good yield