5775 Years in Terms of 77 "Generations"
The following Hiddush should be read with an open mind. It should not be taken as Torah MiSinai and I have no mesorah for it, but it could explain a few things if you keep your mind open. Please comment with your He'arot whether you like it or not.
I. The Ketz is Based on Generations
Hashem told Avraham Avinu regarding the Ketz from the Egyptian Exile (Bereishit 15:16): ודור רביעי ישובו הנה [And in the fourth generation they shall come back hither].
The Mishna in Eduyot 2:9 says:
There is a difference of opinion on whether this part of the Mishna is a continuation of the first part of that Mishna or a separate statement, but the Tosefot Yom Tov quotes the Ra'avad as saying that it's a separate statement that is saying that the true Ketz is based on generations - not years. Going further is the Melechet Shelomo's quote from Rav Shelomo Sirillo, who extrapolates that the Ketz of Mashiah is based on generations and not years (ע"ש).ובמספר דורות לפניו הוא הקץ--שנאמר "קורא הדורות מראש" (ישעיהו מא,ד); אף על פי שנאמר "ועבדום, ועינו אותם--ארבע מאות, שנה" (בראשית טו,יג), אמר "ודור רביעי, ישובו הנה" (בראשית טו,טז).And in the number of generations in front of him is the Ketz, as it says "He that called the generations from the beginning". Even though it says "and shall serve them; and they shall afflict them four hundred years", He said, "And in the fourth generation they shall come back hither".
II. How long is a "Generation"? 75 Years
A. Average from the First Half of History
The Abravanel on the verse from Ha'azinu: בינו שנות דור ודור ["consider the years of many generations"] tells us that this verse is actually telling us look at the years made up of each generation. As we know from Bereishit, people lived very long lives in the very beginning of history. Not only that, but the ages in which they had their children was much higher than it is presently. For example, Adam Harishon had Shet when he was 130. One generation in the time of Adam Harishon was, therefore, 130 years. Later, the years per generation decreased dramatically.
As many Sefarim say, the word אדם is Rashei Teivot for אדם דוד משיח (Adam, David, Mashiah). This tells us that the world has a beginning, a middle, and an end and these figures are significant in these 3 stages. The middle figure is King David, who was born in the year 2854 and lived 70 years.
(The next 2 sources come from the book Mishmar Halevi.) It can be inferred from Rashi on Tehilim 61:7 that he holds that a generation for David Hamelech is 35 years. However, Ibn Ezra on Iyov 42:16 used that verse against the idea that a generation is 35 years. In addition, the Ibn Ezra says that King David was 11 generations after Yaakov Avinu, while Heiman, who lived at the exact same time as King David, was double that (i.e., 22 generations).
Let's list the generations from the beginning of Creation until the time of King David:
There are 33 generations from Adam until 35 years after King David was born, which is a generation for King David, as Rashi explained. 2854 + 35 = 2889. If we now take the average generation from Creation until 2889, we'd calculate 2889 / 33, which equals 87.55 years.
Heiman already had children who sang and was the head of the household when he was appointed by King David to sing (see Divrei Hayamim Alef 6:18 and Radak there). Let's say that the end of his generation could be around the same year - in 2889. To get the average generation from Creation until 2889, we'd calculate 2889 / 44, which equals 65.66 years.
King David and Heiman were the 2 extremes, as per the understanding of the Ibn Ezra. To now get the average generation, we now average out the 2 extremes of 87.55 years and 65.66 years and we get to 76.6 years. Being that Bo'az was extremely old (some say 400 years old!) when he married Rut, it would seem to vastly skew the average and we can therefore subtract another year and a half or so to make an even 75 years.
All this tells me that from the time of Creation until the time of King David, an average generation is approximately 75 years. We are not yet counting the generations, but telling how long a generation should last based on the first half of history and applying that number throughout history.
B. Based on a Lifespan
Although most people would consider a "generation" to mean the amount of years from father's birth until son's birth, there is another opinion as well. The Ibn Ezra on Bereishit 15:16 says that the measurement of a generation is ומדתו הקץ שידור אדם בחלדו - the end of time of man's living on earth, meaning a lifespan. The Radak in Sefer Hashorashim explains it similarly.
The verse in Tehilim (90:10) says that a normal lifespan is 70 years, but if someone is stronger, 80 years. The average between a weaker person and a stronger person is 75 years.
III. 77 "Generations" until Mashiah?
Let's now take a look at Bereishit 4:24:
According to the simple explanation of this verse, Lemech was saying a Kal VaHomer that if Kayin, who purposely murdered, could remain alive for 7 generations, Lemech, who killed accidentally (or acc. to some Midrashim, in self-defense), should be able to remain alive for 77 generations.כִּי שִׁבְעָתַיִם, יֻקַּם-קָיִן; וְלֶמֶךְ, שִׁבְעִים וְשִׁבְעָהIf Cain shall be avenged sevenfold, truly Lamech seventy and sevenfold.
If one generation is 75 years, 77 generations is 5775 years.
Interestingly, the Tanhuma on this verse says that at this point, Lemech became the Malach Hamavet. This could possibly mean that after 77 generations, i.e., 5775 years, the Malach Hamavet will be gone. (Even though the simple explanation of the Midrash is that Lemech became the Malach Hamavet after the 130 years that Kayin was the Malach Hamavet, it's possible to say that it means 77 generations total from Creation.)
Also, interestingly, the very next verse (Bereishit 4:25) is:
and the Midrash Rabbah 23:7 explains on this verse that when Shet was born, Havah foresaw the Mashiah being born.וַיֵּדַע אָדָם עוֹד, אֶת-אִשְׁתּוֹ, וַתֵּלֶד בֵּן, וַתִּקְרָא אֶת-שְׁמוֹ שֵׁת: כִּי שָׁת-לִי אֱלֹהִים, זֶרַע אַחֵר--תַּחַת הֶבֶל, כִּי הֲרָגוֹ קָיִןAnd Adam knew his wife again; and she bore a son, and called his name Seth: 'for God hath appointed me another seed instead of Abel; for Cain slew him.'
May the year 5775 bring a slaughtering of the Satan - who is the Malach Hamavet - and bring us the coming of our King Mashiah with mercy, bimheira veyameinu, Amen.