Most people will use it the last two weeks of flowering to flush and use molasses and water only. Some use it during the whole flowering cycle with nutrients and flush with molasses only at the end. Some people use it all throughout the life cycle.
Greenleaf...You've not mentioned if this grow is inside under lights or out of doors in the ground/pots. I've always 'started' with 2 TBS/gal. It will supply most of the P and K you'll need, along with micro nutrients that assist with the intake of all other nutrients. The last two weeks of flowering, I increase to 3TBS/gal. You an almost watch your buds swell; so fat and sweet. You'll not have a more mild and smooth smoke after a proper cure. I've included a site with the most definitive and detailed explanation for the wonders of molasses...B
I used it for one grow during the flowering cycle. I wasn't impressed. And I thought it changed the taste of the MJ funky taste ,I didn't care for it at all.