In my opinion, best time to flip is when the plant is roughly half the size that you want it to be in the end. If you have 3ft growing space, flip at 1.5 ft tall. This is to account for stretch. Approx I know but seems to work out well. I think you'll find a few sources that say pretty much the same. Especially a couple or few weeks into flower, some strains can really shoot up fast! Had this problem with my grapefruit diesel plants I have at the moment, which is the reason I had to write a new thread asking if kinked stems were a problem. Trust me... The heart ache when you feel a stem bust in your hand...:'( luckily I think they'll be ok but the point is don't let it happen to you! Hope this helps!