When to turn to flowering.

When about half the area is filled... if other issues limit your ability to do it right, do what you gotta do. - Other options include super cropping or whatever dumb label is made for simply breaking stems down to a lower level and allowing them to heal -- viable as longn as branches aren't so brittle they snap off completely.. comes with some risk. Using a scrog in future can help save a lot of vertical space too. level it out at a point where it will have room to grow vertically at a time of your choosing.

The humidity from the top of the soil to the top of the pot must be incredibly high in there; fill the pot to nearly the top. The high humidity is destroying any real growth. The plant has been grown with a massive shortage of oxygen. The pocket barrier of high humidity above the soil will prevent efficient oxygenation of the soil; in a plastic/ceramic pot, it's practically a death sentence for meaningful growth indoors. If it took 12 weeks to get to that mass, there is not much point in flipping until your plant is actually converting a reasonable amount each cycle. Possibly the first plant I've ever seen grown entirely using anaerobic fermentation and still be alive & kicking; that's what happens when you go from 38 ATP per glucose molecule and drop down to 2 ATP When you leave a deep, empty lip at the top of a pot, you create a stagnant "microclimate" trap. Growing for 7 years, dude. Need to get back to basics. Best of luck.