Midrasch zu Schemot 7:25

12. "And a full seven days were completed after Ad-nai struck the [Nile] River" - Rabbi Yehudah and Rabbi Nechemiah (disagree). One says he warned them for 24 days so that the plague wouldn't come, and seven days they experienced the plague. The other says for seven days he warned them, and for 24 days they experienced the plague. According to the opinion of the one who says for 24 days he warned, and the plague took 7 days; and according to the opinion of the one who says for seven days he warned and the plague took 24 days, [we find that] "a full seven days were completed after etc" that he warned them regarding the next plague.

How long did the plagues last among them? R. Johanan and R. Nehemiah discussed this question. The former said: He warned them for twenty-four days before inflicting a plague upon them, but the plague lasted seven days, as it is said: And seven days were fulfilled, after that the Lord had smitten the river (Exod. 7:25). The latter held that he warned them for seven days, while the plague itself continued for twenty-four days.