Whenever you want to boost your microbe population on top of nutes. Most recommend around 1 - 2 Tablespoons per Gallon. It it used in teas a lot for the reason of increasing microbe populations essentially adding it with your water will provide the same effect once added to the soil with a microbe population. You are using Fox Farm Ocean and Worm castings which all contain microbes so whenever you are ready go for it. I would suggest a little amount first till it fills in the pot more. I tend to use it at least once a week with mine however i am in flower atm. The plant won't use the molasses itself the microbes break it down so the plant can use it in return they get the sugars from the plant. Good luck with your grow.

I wouldn’t add anything until after week 5. My understanding of adding molasses you should also add some sort of myco to make sure there is definitely something there to eat those extra sugars

That's great you're going all natural with molasses! I know it sure sweets up and bulk up those buds. From all I've read, adding about a half a tsp per gallon each feeding in the last 3-4 weeks of flowering works good. I don't even think you have to worry about flushing it out like other nutes and supplements as much. Happy growing I hope those new lights really boost your grow! More good quality safe distance light is always good, almost as good as having good genetics.

Whenever you want, you can even work with molasses before the vegetative stage, when you’re ‘cooking’ your soil. But in most cases, the benefits seem to happen during the flowering stage, that’s why many growers use it when plant shows it first pre-flowering signs. I’ll be around, good luck with these babies!