Rashi on Exodus 7:25/רש"י על שמות ז׳:כ״ה

1וימלא AND THERE WAS COMPLETED (the verb is singular) שבעת ימים the number of SEVEN DAYS during which the river did not return to its original condition. For each plague functioned a quarter of a month and for three quarters He warned and cautioned them (Midrash Tanchuma, Vaera 13; Exodus Rabbah 9:12).