How long into the flowering phase do plants stop stretching?

That's a good question, I had Autoflowering plant veging for 9 weeks and stretching for first 5. On average they go longer than photo dependent , plus cola height always increases if you do it right. Your plant tells me you do ! There is SuperCrop technique. Google it, I use it a lot and never top, with autos I think it's best way, especially when no space left . Just do it smart, prethink how everything will look after, do it before watering, work 5-7 cm of stem before letting him bent, give support for not fall down, but not to stretch back too and you should win space and increase harvest even. Give side crowns shoots light touch too, push them upwards, you will have few more colas to smoke ;)) All the Best !

Usually between 2-4 weeks.