How many centuries do historians think it took from the time Islam was introduced until it became the religion of the majority population in Egypt Syria Iran and Spain?

Answer 1

It is very different in all of these different countries, because the process of conversion moved at different rates in different countries. This is because while there was pressure to convert to Islam, forced conversions were rare.

In the case of Iran, it became Muslim-majority as early the mid-800s, roughly 200 years after Islamic conquest of the region. This was primarily due to restrictions requiring all bureaucrats to be Muslim and prohibitions within Zoroastrianism to work in medieval forges (which made conversion to Islam more desirable).

In the case of Egypt, it became Muslim-majority by the mid-900s CE, roughly 300 years after Islamic conquest of the region.

In the case of Syria, it became Muslim-majority by the year 1100 C.E., roughlt 450 years after Islamic conquest of the region.

In the case of Spain, it never became Muslim-majority. Even while Muslims were ruling the Peninsula, conversion by endemic Iberians, who were the overwhelming majority population, was slow. Additionally, Spain included territories under Christian Rule, which strongly retarded the growth of Islam in those areas.