“The Orthodox may have somewhat earlier, though not numerous, beliefs in the assumption but even then it was not until the 4th century that it saw the light of day.”
But since the 300s, we have, virtually all of us, believed firmly that the Most Holy Theotokos was assumed bodily into heaven. As I said, however, this is NOT dogma in The Church in the East and thus all of us are free not to believe it.
Here is a late (14th century) sermon on the Dormition by one of the greatest Fathers of The Church, +Gregory Palamas. I post it to you because it is best sermon on the doctrine I am aware of and it is particularly scriptural.
http://www.ocf.org/OrthodoxPage/reading/dormition.html
As I said; 4th century.
You have believed since the 300's? Wow! And I thought I was old. :-)