Sforno on Exodus 10:3/ספורנו על שמות י׳:ג׳

1עד מתי מענת, seeing that in the meantime I have demonstrated to you My dominance even in the atmosphere, how much longer will you refuse to humble yourself seeing that I control the very air you need to breathe? Seeing that you were not impressed, there is no sense in hoping that you will be impressed by the overwhelming show of strength represented by the plagues. However, perhaps if the impact of the miracle itself did not make an impression on you, perhaps the drawn out duration of the effect of the plagues may finally have the desired effect. This is why the question “how long must a plague continue before it will have the desired effect?” is not out of place.