Shemos 7:25 And seven days were complete, after G-d smote the river. Rashi explains that each makah lasted seven days, and Moshe warned Paroh for the rest of the month (three weeks). Ibn Ezra and Rashbam argue and say the makah of blood lasted a week because the verse tells us so, but there is no reason the think that all the other plagues lasted seven days.
(Everyone seems to agree Makas Bechoros happened at once, and did not last seven days. Regarding darkness - three days of darkness and then three days of thick darkness that inhibited movement - that's less than seven days! Sifsei Chachamim discusses it. Regarding Dever - from Rashi to 14:7, the righteous Egyptians' animals did not die. Rashi to 9:10 explains that the animals that were kept indoors did not die. It can therefore be that the plague lasted seven days even according to Rashi, for if animals were indoors at the beginning of the plague, if they came outdoors during the seven days, they would die. (about dever was mipi Mori Shlit"a answering my question.))
What is the significance of seven days that Rashi holds all of the plagues lasted that long? Rambam in Moreh Nevuchim 3:43 discusses why do Pesach and Sukkos last seven days? He says because seven days is long enough to realize that something was different. Not eating bread for a couple of days isn't so out of the ordinary. But when you don't eat bread for seven days you realize it happened, and you also realize why - because of the mitzva. We can likewise explain the duration of the makos similarly. Had they only been a day or two, it was likely that some people didn't realize anything happened. But because they all lasted a week, everyone in the entire Egypt realized that all of these miracles took place.
With this we can understand another Rashi. The Gemara (Kiddushin 30a) says we must split our learning into thirds - a third of the time we must learn Tanach, a third must be Mishna and a third must be Talmud. Rashi (s.v. leyomi) explains that it means every week we split up our learning, so that two days are Tanach, two are Mishna and two are Talmud. Tosfos (s.v. Lo) finds this difficult and explains that every day we must learn all three, so that our daily learning is split up three ways. Rambam (Hil. Talmud Torah 1:11-12) agrees with Tosfos. Tosfos has a good point with his question against Rashi. Why, then, does Rashi not say we must split our daily learning into thirds? He holds that as long as you did something within seven days, it is considered to be current. We know one might get involved and engrossed in a certain topic for a couple of days. Doing so does not indicate that he is ignoring the other two parts of Torah. But engrossing in one of the three parts for a week does indicate that you are ignoring the other two parts. Therefore Rashi says the learning only must be split up weekly, but not daily.
There is another answer we can give for Rashi. The Gemara asked how do you split up your learning into thirds, for how do you know when you will die? Rashi says you split your weeks in to thirds - two days, two days, two days, but Tosfos still asks, maybe you will die before the week is complete? The Shu"t Kesav Sofer O"C 103 brings a Chacham Tzvi 106 who shows from Tosfos Gittin 28a, Rosh Nedarim 3b and Gemara Sukkah 23b that there is a machlokes Rashi and Tosfos if we are worried that one may die within a week. Tosfos has such a concern, but Rashi does not. Thus, here, in regard to splitting your learning into thirds, Tosfos is worried you may die within a week and not have learned all three parts of Mikra, Mishna and Gemara, but Rashi is not worried about dying within one week.
(With this Ksav Sofer we can explain the machlokes Rabi Yehuda/Chachamim at the beginning of Yuma if they need to appoint a back-up wife for the Kohen gadol lest she dies before Yom Kippur, and the Kohen Gadol must be married: R"Y holds like Tosfos and we're worried she will die within a week, so we appoint a back-up wife, but Chachamim hold like Rashi and we're not worried the kohen gadol's wife will die.)
Tosfos gives some other ways how we fulfill our need to learn all three parts of the Torah daily. One is Rabbeinu Tam, who explained that Talmud Bavli is like balul, a mixture, of all three, so by learning Talmud Bavli you fulfill learning all three. Rav Amram Gaon says that is why we say psukim, mishnayos and the braissa of Rabi Yishmael every morning before davening - to have learned a little of all three parts every day.