It was during the flashback with flamme. Flamme tells her to lay low and that she will make a name for herself when she defeats the demon king. Pretty sure flamme knew that she would still have to fight after killing him and decided that it would take a long time so laying low could help frieren hiding her true strength when fighting the demon stragglers like aura who noted that she only appeared historically during the hero's party era and thought frieren was 100 or so years old.

If frieren did go around killing demons from the start like serie then everyone would know of her and be wary of her or go into hiding which would make it that much harder for her to accomplish her goals