I second what Russrahl is saying. Stretch is going to be very dependent on the strain, as well as light height and some other factors. Sometimes its a matter of just flipping and seeing how it goes. It's good practice to plan for a significant stretch. Lots of times it's roughly doubling in size. Once you have a basis for a strains stretch you can plan in the future to flip earlier and train in the earlier weeks of flower. Good luck with the rest of your grow, it's looking beautiful!