The key to understanding what (not who) the "Sea Beast with 10H" of Rev. 13 is, a conglomoration from the remnants of the 3 previous Beast empires before, as shown in Daniel 7.
Rev. 13[1] And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy.
[2] And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority.
That is the Roman Empire!
[3] And I saw one of his heads as it were wounded to death; and his deadly wound was healed: and all the world wondered after the beast.
Now, which of the heads was wounded to death, and was healed?
The clue to us is the 8th head. The Beast is a conglomoration, a compilation of the 7 heads (governments) of the previous 3 Beasts.
In other words, its make up is from all of the 3 beasts prior to it. It became the 4th Beast of Daniel, the Roman Empire, and was/is the 8th head, as shown in Revelation 17[11] And the beast that was, and is not, even he is the eighth, and is of the seven, and goeth into perdition* [destruction]*.
The length of time for this Beast is very long, because it has evolved into many forms over the centuries, since its beginnings as the 4th Beast in Daniel. They are:
1. The Roman Empire, then split into
2. The Roman Catholic church, then dispersed into
3. Europe (EU), and has now generated out of itself
4. The Global Financial Empire, of which shall acquire for its well being and protection, a world army, to be formed out of 10 countries, aka the 10 horns.
*So, when was the Beast, the 8th head, wounded by a sword (war-"perdition") and did live/survive?
Ans. When it was in the stage as Europe, enduring WW1&2.