There is no answer to that question, because it all depends on how you inject the CO2 and how much light intensity you have, plus how much plant mass you have and how fast the plants are growing. Plus, of course, how much CO2 you are losing from the water surface. It is infinitely better to just measure the amount of CO2 in the water, using a drop checker, and go by that.

Dennerle should know better than to make that suggestion. They are the only company I know of that understands how the drop checker should be used, and they sell theirs with the reference KH solution and indicator reagent included.