How long did Muslims persecute Christians?

Answer 1

This depends on the definition of "persecution".

Persecution as Inequality
If persecution strictly refers to any legal, social, political, or military inequality, then every single Islamic Empire since the Rightly-Guided Caliphates have persecuted Christians. Christians were considered Dhimmis or non-Muslims under Muslim occupation. They were required to pay a number of taxes that were connected with the Dhimmi status. The most famous was the jizya, which was a tax that Dhimmi had to pay for Muslims for the right to not be killed where they stood for not acknowledging Mohammed's Prophecy; it was a form of humiliation. Additional taxes included the kharaj, which was a tax on non-Muslim land-holdings in the Muslim World. The kharaj was so untenable that most Dhimmi were forced to live in the cities where the tax would not be applicable. There was also inequality concerning the justice system. On paper, a Christian or Jew could testify against a Muslim, but in reality, such testimony was not acceptable and the attempt to defame a Muslim would receive retribution. Christians and Jews were not allowed to build new houses of worship, restore old houses of worship, proselytize in any way (this included religious debate or dialogue), or allow wine or pigs to be shown in public. In some empires, like the Ottoman Empire, some Christians would be forcibly removed from their families, converted to Islam, and brought up in the sultan's care in order to serve the empire (a system called devşirme).

The legal and social inequality between Muslims and non-Muslims (like Christians) persisted up to the colonial period. Since independence, Jews and Christians who remained in the Islamic World garnered a more equal status than anything that they had previously had under Muslim leadership, but still are unequal in terms of their inability to proselytize, the unofficial "requirement" to avoid offending Islam in public, and the need to seek the authority of high government officials to build new houses of worship or to repair existing ones.

Persecution as Violence
If prosecution only refers to the use of violent acts to compel belief or flight, the Islamic persecution of Christians was very rare. Generally, as long as Christians were willing to accept the humiliating Dhimmi Status, they were allowed to live in peace. Prior to European colonization, the major events of Christian persecution in the Islamic World were typically tied to a political cause. For example, Christians were banned from making pilgrimages to Jerusalem because of the political ambitions of European Christian Rulers to reconquer Jerusalem from Türkic Muslim Rulers.

Since the shift towards legal equality after the European colonial period, Islamic supremacism became more pronounced. Since such a supremacism could no longer have the clear and unambiguous representation of the Dhimmi System, violence and persecution of Christians rose significantly, especially since Christians in the Islamic World were often seen as surrogates of the European colonizers. Persecution has been especially harsh recently in the Islamic State (between Iraq and Syria), but has also been noted in post-independence Muslim-majority countries like Lebanon, Pakistan, Turkey, Egypt, Palestine, Iraq, Iran, Indonesia, and several other countries.