when to flush?

I started my 12/12 cycle on 1/5. Plants started showing at about 14 days. So if i start counting flowering days from the time they have showed, they've been flowering for about 6 weeks. The strain i have says flowering time 9 weeks. So my question is, when should i begin to flush? I need to change my water today, but would now be to early to start flushing

flush for last 2 weeks of flowering

When it says nine week finishing time, that's a rough guesstimate under ideal conditions. See how they look at the nine week mark, my guess is they'll easily go 10-12 weeks before being ready to flush.