Picture for a minute what it would have been like the first time that one of the Apostles realized that the Blessed Virgin was sinless. I would guess that most, if not all of them, would have seen some hints of this amazing truth during Jesus' ministry. Was it completely revealed to them then? Catholic dogma does not tell us definitively.
Imagine, for example, a certain Catholic in the first century who lives near to St. John and Mother Mary, not long after the Ascension of Jesus. He's meditating on the fulness of what it means for Jesus to be Divine. Then he speculates about our Lord's sinless flesh. Next he realizes that this sinless flesh came from the womb of the Mother of Jesus. Then, "ding!", a light goes on in his mind and he turns to look at her with a new perspective; shock, awe, and a bewilderment.
Did it happen this way? It very well could have for many of the early faithful. Consider the realization of the Immaculate Conception and let it sink in that it is never specifically mentioned in the Scriptures so it was not necessarily in the forefront of the Apostles' minds. It is certainly possible (and likely) that Jesus explained it to the Apostles before His Ascension into Heaven. Either way, the Church came to grasp this truth as a clear testimony of the work of God in the process of our salvation. While the Immaculate Conception is not the center of our salvation (Christ is), it is crucial to how our salvation works out.
This is exactly how many of the details of the faith come to people who are considering becoming Catholic; they take baby steps and then come to accept a "new" (to them) concept. They suddenly take a leap forward in their faith journey and see everything in a new light. It happens often with the doctrines of Mother Mary, and I have been there at that very moment for a few people; it is a wonderful joy to see it. It only happens, however, if our hearts are open to God teaching us His truth. Is your heart open to learning?