Yes, if your FC yo-yos it takes much longer.

Is your pool vinyl or hard-sided? If it's vinyl, I would keep FC at 25ppm, not 20, and check it 3x/day. If it's concrete or plaster or something else hard, if it was me, I would take FC up to 30 or 35ppm and keep it there--won't damage a concrete pool but will bleach a vinyl liner. I'd really hammer the algae. But the decision is yours. Whether it's vinyl or concrete, I would at least take FC up to 25 instead of 20 and keep it there.

Carl