[Avodah] what did we learn for 350 years?

[Moshe Y. Gluck]

R' AY & CB Walters:
The Geonim say that from the chasimas hatalmud by R' Ashi until the Behag
was 350 years.

Now, AFAIK, the Behag was the first post talmudic sefer, he was probably
from the early gaonim/late saboraim.
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I believe the She'iltos D'R' Achai Gaon preceeded him.  

KT,
MYG