In the Book of Revelation, when is Christ coming back on a cloud, before or after the tribulation?
In premillennialist doctrine, there is a reference to a "great tribulation," but the various believers in premillennialism cannot make up their minds about the details. Some claim that Jesus will come and take the righteous from the earth. This leaves no restraints on the wicked so they have a hay-day for a period of time. Jesus then returns for the third time and establishes order, reigning for a thousand years. Others claim that Jesus comes and reigns for a thousand years, but at the end of his reign Satan leads a force against Jesus and appears to gain the upper hand for a period of time, but then Jesus ultimately conquers and leads the righteous off to heaven.
The problem is that neither view is taught in the Bible. Peter is quite clear that Jesus' return means the end of the world. "But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up" (II Peter 3:10). There are not two separate resurrections: one for the righteous and one for the wicked, but just one. "Do not marvel at this; for the hour is coming in which all who are in the graves will hear His voice and come forth--those who have done good, to the resurrection of life, and those who have done evil, to the resurrection of condemnation" (John 5:28-29). See "An Overview of Premillennial Theories" for more details.
But you asked about the tribulation mentioned in Revelation. Let's look at each verse where it is mentioned.
"I, John, both your brother and companion in the tribulation and kingdom and patience of Jesus Christ, was on the island that is called Patmos for the word of God and for the testimony of Jesus Christ" (Revelation 1:7).
Rather than speaking of a future event, John speaks of the tribulation as something that has already begun and is currently continuing. John was a partaker with others in this tribulation.
"I know your works, tribulation, and poverty (but you are rich); and I know the blasphemy of those who say they are Jews and are not, but are a synagogue of Satan. Do not fear any of those things which you are about to suffer. Indeed, the devil is about to throw some of you into prison, that you may be tested, and you will have tribulation ten days. Be faithful until death, and I will give you the crown of life" (Revelation 2:9).
In speaking to the church in Smyrna, Jesus states they have been facing tribulation. Again, it was something occurring at the time of the writing of Revelation -- not something in the distant future. This tribulation would get worse, but it would only last a short period of time.
"Indeed I will cast her into a sickbed, and those who commit adultery with her into great tribulation, unless they repent of their deeds" (Revelation 2:22).
Jesus told the church in Thyatira that if they did not clean up their act that he would visit tribulation upon them because of their sins. Again, this cannot be something in the far distant future. If it was, the threat would make no impact on those committing these sinful deeds.
"And I said to him, "Sir, you know." So he said to me, "These are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb" (Revelation 7:14).
At the beginning of Revelation, John stated, "The Revelation of Jesus Christ, which God gave Him to show His servants--things which must shortly take place" (Revelation 1:1). One of the things John saw were martyrs (Revelation 6:9). His vision showed that they were not permanently killed. They continued to live and were with God. They had survived the tribulation that killed them -- not on earth, but eternally in heaven. Again we see references to something that is ongoing.
The tribulation in Revelation is a reference to the great persecution of the church by the Roman government; a persecution that had already begun when John wrote Revelation and was going to continue for a while. Paul had mentioned that the persecution had begun or would soon begin when he gave his advice about marriage. "I suppose therefore that this is good because of the present distress--that it is good for a man to remain as he is" (I Corinthians 7:26). From history, we know that this started under Nero when Paul lived and became incredibly severe under later emperors, such as Domitian. The tales of the early Christian martyrs come from this era.