It's more a question of when than how often.
Great White is not a nutrient. It contains beneficial microorganisms (mycorrhizal fungi) that can grow their own numbers, so it only needs to be added once. The microorganisms live in symbiosis with the plant's roots and help create a healthy rhizosphere that is the essence of organic growing.
It's best to follow label instructions for use.
Great White can be added several ways, such as part of a top dressing or it can be watered in. Only a small amount of
Great White is needed. I use about 1/8 teaspoon.
Mykos is similar but less flexible because it needs to be applied to the roots when transplanting. Some starter soils also contain these beneficial microorganisms.