The oldest book of the Bible is the Book of Job. In chapter 1 of Job, we read how Satan (another name for the Devil) comes before God, suggesting to God that this man Job is not genuine in his love for God. In the scene, the “sons of God” are also present, which we take to refer to the angels. In as much as we believe that God is a literal being, and that Job is a literal person, and that all of the angels are real, we have no reason to believe that the Devil is not. Now, some people think that there was no such man as Job. The book is somewhat of a theological discussion between Job and his friends, and some people feel that this entire book is hypothetical. So, according to the view, this story at the beginning never really happened, it just sets up a context for the theological discussion. But even if this were true, this hypothetical discussion is about the reality of God and the angels. Later in the book, as part of this theological discussion, it talks about the time of Creation when “all the sons of God shouted for joy.” (Job 38:7).
That passage in Job 38 doesn't really give us a definite time that the angels were created, but does seem to suggest that they were at least created before mankind. From elsewhere in the Bible, we understand that some of the angels rebelled against God, and that Hell was prepared as a punishment for them (2 Peter 2:4, Matthew 25:41). At what time this took place, the Biblical narrative of Genesis appears to be silent. But in Genesis 3, of course, the Serpent is clearly understood to be Satan. The Serpent comes and tempts Adam and Eve to sin through disobeying God. Now, Romans 5:12 says “sin came into the world through one man”. Since sin came into the word through the sin of Adam, it seems logical to me that until Adam ate the apple there was no sin in the world. That is, not even the Devil had sinned yet. Therefore, the answer to the question “When did Satan fall?” is possibly “Genesis 3, along with Adam and Eve.” Since Romans 5 considers Adam and Eve to be literal people, and the story of how sin entered the world to be a true, historical story, then it would follow that the Devil, as a main character in this story, is also a literal figure in the story.
The Old Testament doesn't speak much more of Satan directly, yet he's not forgotten. In 1 Chronicles 21:1, Satan is directly identified as tempting David in a certain sin. In Zechariah 3 we read of Satan opposing Israel's progress in rebuilding the nation. In Daniel we read of how the angel Gabriel was delayed in reaching Daniel because he was engaged in some kind of battle with an opposing spiritual being; whether the Devil himself or one of his angels. But when we get to the New Testament, the Bible has a lot to say about the reality of the Devil. Our first major New Testament Text is the temptation of Christ in the wilderness, in which the Devil tried to cause Jesus to sin through various temptations. And of course, this alludes right back to Genesis 3 where the Devil tempted Adam to sin. Whereas Adam failed, Christ would not. But we also find a parallel between this story and the wilderness wanderings of Israel. As Jesus is tempted in the wilderness, He continues to quote the Book of Deuteronomy against the Devil. Each of the passages He quotes are used in Deuteronomy to speak of how Israel failed to be faithful and obedient to God. It seems, then, that in the use of this parallelism, the Bible is saying that the Devil is tempting Jesus in the same ways that he tempted Israel. Again, where Israel failed, Jesus would not. We understand, then, that the Devil was behind the temptations that Israel fell into. And later in Israel's history they worshiped many idols, which the Psalmist tells us were ultimately demons, servants of the Devil; “They sacrificed their sons and their daughters to the demons” (Psalm 106:37). Richard Dawkins likes to say that we are all atheists toward other gods but our own, but actually the Bible says that the false gods which Israel worshipped were quite real. But they are not the God who created heaven and earth, and who saves us from our sins.
Later in the New Testament, Peter writes “Your adversary the devil prowls around like a roaring lion, seeking someone to devour.” (1 Peter 5:8). Peter himself had been tempted by the Devil to deny Christ; which Peter unfortunately did. Jesus had even warned him; “Simon, Simon, [that is, Simon-Peter] behold, Satan demanded to have you, that he might sift you like wheat, but I have prayed for you that your faith may not fail. And when you have turned again, strengthen your brothers.” (Luke 22:31). It's worth noting how Peter gives us this warning about the Devil in 1 Peter 5:8, having gone through such an experience. Jesus never prayed that the Devil would leave Peter alone, or that Peter would not fall into temptation, but that Peter's faith would not fail. Because Jesus had prayed that Peter's faith would not fail, He could say with confidence “When you have turned again...” In God's wisdom, we are not kept from the temptations of the Devil, or from being harassed by him, as Paul writes in 2 Corinthians 12. But Jesus keeps our faith from failing; through Him we persevere to the end. The Devil may afflict us from time to time; God allows the Devil to this, just as He allowed the Devil to physically and psychologically torment Job, because there are lessons to be learned and through such trials our faith is strengthened.
And they have conquered him by the blood of the Lamb and by the word of their testimony, for they loved not their lives even unto death. Therefore, rejoice, O heavens and you who dwell in them! But woe to you, O earth and sea, for the devil has come down to you in great wrath, because he knows that his time is short! (Revelation 12:11-12)