When should I transition my plant to flowering?

I would quickly train your tall parts by pulling the down level. The more level the canopy the better use of light. But apart from that you could switch the light schedule anytime now.

Flip when your plant is just below half of what height you want it to finish at. If you want you plant to be 20" or less, then flip around 8". If you can go 36", then flip around 15". This is for indica and hybrids. Strong sativas can triple in height after flipping, so flip a little earlier with them. With a ScrOG net, you flip when the net is 80% full.

Looks like you are in a small space. I would tie down the upper main stem to make it just below the others and flip in the next 48 hrs.

I think it’s really up to you. Your plant looks healthy and is definitely mature enough. I usually try to wait at least 4 weeks into veg, however this can change depending on how big you want your plant to get. You’re plant definitely could be flipped now! You’ll want to consider your space, because some plants can double or more in height during the first few weeks of flowering. Good luck! Also, I’m a bit of a noob, but this is to the best of my knowledge!