I feed every watering. This will be different for every person in every grow setup.
How much feed and water your plants take is based on several variables.
Light intensity, higher light intensity is faster growth, therefore more nutes needed
Temp, ideal temp is between 78-82 for flowering cannabis, temps bellow this and the plant grows slower, therefore less nutes and water needed.
Humidity also effects growth, in flower go for 50-60% RH. research "vapor pressure deficit" to see how this works.
High temps, with low humidity cause fast water/nutrient uptake and can cause burning with a low nutrient EC. Lower temps and higher humidity slows down nutrient uptake
Hope this helps
Sidd