There's no correct answer to that question, for me it depends on how often you want to buy molasses. That stuff gets expensive. Nowadays I use 1 teaspoon PER GALLON and that's strictly for the microbe tea, super soil day 1, 30, 45 molasses drench.
Allegedly excessive amounts of molasses may lead to N lock-out. Remember to use blackstrap molasses wherever possible because it has the highest concentration of potassium and minerals. Like elduece said, 1 tsp\gal is a good starting point for microbial stimulation in teas. Higher doses stimulate bacteria primarily and can supply some K, still over a tablespoon or two isn't necessary. Earth Juice sells a Hi-Brix molasses for plants with an NPK of 0-0-3, and lists usage rates varying from a teaspoon or less to over a couple tablespoons.
ive gotten up to 2tbl/gal then they were due for a chop. i started at 1 tea/gal and worked my way up to 2/gal. i never tried pushing it more than that but now that its been mentioned im wondering what a whole jar of molasses in a gallon of water would do...nute burn i imagine (since molasses does supply nutes)