When to stop feeding and start flushing?

You have a long way to go. When it starts to look close to ripe it's usually a good time, because there's still 10-14 days, typically, from that point. Being familiar with trichome progression will help too. Start observing, now :) Anyway, your substrate will impact how long you flush. With soil you have extra room for error.. can always changeup midstream if it is going longer. Bring it down slowly though. Don't flood the thing. Normal irrigation with healthy runoff is all until top 1" dries again. you can only pick out the day if you are familiar with the plant. The plant does the planning, regardless.