First of all, let me correct what I said in post #4 - 16 gallons is about 2 cu. ft. of soil, not 2 "bags".
The way I use it, it doesn't "run out". I keep repotting into larger pots, periodically, so there is always a "fresh" layer of soil available to the roots. Since the vast majority of the roots' new growth is "congregated" around the circumferance (and toward the bottom) of the soil in the pot - it all works out very well!
Marine Cuisine is not the only fertilizer I have going on in there! I mix my own soil and it is fully 25% Composted Steer Manure. I mix the Marine Cuisine with two other, similar, "Meal" based fertilizers (not FoxFarms products) at about 1/3 of a dose each. I use this same soil mix from the time the plants (clones) are "a couple of weeks" old - all the way through harvest. During the veg stage I use Mexican bat guano tea and/or 5-1-1 fish emulsion. During the flowering stage I use Indonesian bat guano tea and/or 0-10-10 fish emulsion.