A period of 3½ years is a recurring theme in end-time Bible prophecy. The Beast that is described in the later chapters of Revelation is to receive power and authority from Satan for a period of 42 months (Revelation 13:5). That is the same length of time that gentiles will "tread" Jerusalem underfoot (Revelation 11:2). In Jerusalem, God promises to raise up two prophets to be His witnesses for 1,260 days. God "feeds" or provides for the woman of chapter 12, representing God's chosen people, for 1,260 days (verse 6). We are told she will be "nourished for a time and times and half a time [3½ years], from the presence of the serpent" (Revelation 12:14).
Daniel referred to a coming deceiver as a pompous "little horn" arising among the 10 horns on the fourth beast, warning that "the saints shall be given into his hand [as martyrs] for a time and times and half a time [3½ years]. But the court shall be seated, and they shall take away his dominion, to consume and destroy it forever" (Daniel 7:25-26).
In another end-time prophecy, Daniel speaks of "the power of the holy people" as being shattered for the same period of time—3½ years (Daniel 12:7). This recurring reference certainly appears to be significant, indicating that a total of 3½ years will elapse from the beginning of Satan's attack on the people of God until God destroys his military and religious system and locks him away at Christ's return.
Regarding the 1,290 days and the 1,335 days referred to in Daniel 12:11-12, the context doesn't clearly explain these times. Nor do we have clear understanding of them from another part of the Bible. But we can have confidence that those who wait patiently on God through these terrible times will be blessed in God's soon-coming Kingdom.