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 killed the Bechoros in one moment, but it was seven days from when the Bechoros began killing their fathers (M"Z 19:1).
Da'as Zekenim, 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; the darkness at Keri'as Yam Suf completed the Shi'ur of seven days (also Riva 10:23, citing Midrash Tanchuma).