Almost everything Christians believe about Mary beyond the Gospels was defined centuries later, and the dates are worth having straight because they are routinely collapsed.
Theotokos, God-bearer, was affirmed at the Council of Ephesus in 431. The dispute was not really about her: it was about whether the one born of her was a single person who is God, and Nestorius' preferred title, Christ-bearer, was rejected because it seemed to split him in two. The Marian title was settled as a christological instrument.
Perpetual virginity, that she remained a virgin after the birth, is not stated in the New Testament and is first argued at length in the second-century Protoevangelium of James, which supplies her parents' names, Joachim and Anne, and makes the brothers of Jesus children of Joseph by an earlier marriage. Jerome offered a different solution, making them cousins. The doctrine is held by Catholics and Orthodox and by Luther, Calvin and Zwingli; most later Protestants do not hold it.
The Immaculate Conception, that Mary herself was conceived free of original sin, was defined by Pius IX in 1854, and is very often confused with the virginal conception of Jesus, which is a different thing entirely. Thomas Aquinas argued against it. It is not held by the Orthodox, who frame the question differently.
The Assumption was defined by Pius XII in 1950, the only exercise of papal infallibility since the doctrine was itself defined in 1870. The Orthodox keep a related feast, the Dormition, without the Roman definition.
Nineteen centuries separate the earliest of these from the latest.