Or HaChaim on Exodus 7:25/אור החיים על שמות ז׳:כ״ה

1 וימלא שבעת ימים, Seven days went by, etc. The word וימלא, "became full," refers to the pre-arranged length G'd had ordained for the plague. After that period had elapsed, G'd told Moses to go and see Pharaoh again (verse 26). The Torah tells us this detail to show us the obstinacy of Pharaoh who could let an entire week go by without making an effort to somehow have this plague terminated. All of this is in accordance with an opinion expressed in Shemot Rabbah 9,12 that Moses spent 24 days warning Pharaoh of the impending plague whereas once the plague materialised it remained in effect for seven days. According to the opinion that it was the other way around, i.e. that Moses warned Pharaoh of the impending plague for seven days running, whereas once the plague materialised it lasted for 24 days, the seven days mentioned here refer to the period of warning. The plague then would have remained in effect for the remainder of the month. The problem with this explanation is the plague of darkness which is described as being intense for three days (10,22). Perhaps both the plague of darkness and the plague of blood were exceptions and the author of the opinion that the plagues remained in effect for 24 days agrees that the water in the river did not remain blood for 24 days on end. This would account for the fact that the Torah mentions a time frame only in the case of those two plagues. I believe that even though G'd had set a time-limit for each plague, Moses' prayer would have sufficed to terminate the plague sooner.