It depends on how big they are and how hungry and the amount you feed them .
A safe answer would be once a week 3 TBSP per 5 gallon and 1 tsp Epsom salt per gallon and either go up from there in the amount of food used or to twice a week .
These are things a grower knows by reading their plants .
I have 3 Strains I am currently growing. Sour Diesel , GSC . & Durban Poison .
All of my girls are in the starting stages of Bloom .
Planted May 30th , Souther Oregon
I am currently feeding my plants 1 TS / 5 ML a gallon per plant . Smaller plants i use half the dose
I have seen some slight nute burn , nothing serious . However , I think I need to mix it in with more water to be safe .
Some of the bigger plants are 7 ft / 8ft tall right now . they are in need of fertzs big time