Assuming 2x/week watering...
1/8 teaspoon? 1/4? 1/2? 1 full tsp?
Depends on the plant, and depends on the time of year mostly.
With my peppers *now* as the season is winding down, 1/2 teaspoon in a gallon, twice a week.
For more tailored advice, let us know what the plants are.
Light feeders like Camellias, Gardenias and Jasmines.