What is the significance of the seven days mentioned here?
Ramban: This refers to the period of time the Egyptians spent digging round the river searching for water.
Rashbam and Targum Yonasan: The plague of blood lasted seven days. After seven days, Hashem cured the water of the river.
Moshav Zekenim: 'Nezifah' (censure) is not less than seven days - Mo'ed Katan 16a. Hashem actually killed the Bechoros in one moment; however, it was seven days from when the Bechoros began killing their fathers (M"Z 19:1).
Da'as Zekenim and Hadar Zekenim: There were 30 days between Makos, like the Gematriya of the extra Chaf-Yud at the end of "Shalach Osos u'Mofsim b'Sochechi Mitzrayim". Choshech was six days and the darkness at Keri'as Yam Suf completed the Shi'ur of seven days (also Riva 10:23, citing Midrash Tanchuma).