Day by day. Herein is the strategy of temptation. We may also see another possible implication in the text as well. This would show temptation to be also most subtle. As the serpent tempted Eve with subtlety, so too doth the harlot tempt Joseph.
Temptation is subtle, working by degrees to cause a soul's destruction .
Furthermore, Potiphar's wife did not so much wish to seduce Joseph, but rather she wanted to corrupt Joseph. Therefore, all temptation is a effort to corrupt through seduction unto destruction.