I used 1 TBSP per 4 gallons, but that wasn't really helping my buds swell, so I kicked it up to 1oz per 4 gallons and am seeing improvement, that's with a 'robust' or blackstrap molasses. I used to use 1 TBSP a gallon, but that was making my soil go sour in pots that weren't well rooted yet. Usually indoors when I cut off all food, I usually still add molasses, and depending on how far I pushed them and how much they are still drinking, I'll do 1 TBSP a gallon because they are starving for anything.
Oh, and molasses totally does awesome things above ground on plants, you can even spray it on your buds if they aren't already swelling good.