I'm sure the bag also states that you should fertilize with miracle grow fertilizer every couple of weeks. It all depends on how much the plants feed through their roots (for our application) and what the demand is.
Low light, no CO2 = slow growth: nutrients will last longer.
High light, lots of CO2 = fast growth: nutrients will be used up faster.
I'm sure dosing the water column lengthens the amount of time that nutrients remain in the substrate unless you dose lightly and have plants that rapidly draw in the nutrients through their leaves so that other plants are relying more on the substrate, or if you have a ton of plants that only feed through their roots.
How long have your other tanks been set up? Are you seeing any deficiencies (assuming that the bulb isn't old and starting to weaken and shift light spectrum and intensity)?