According to Marcus Jastrow and Wilhelm Bacher in the Jewish Encyclopedia entry for "Ashi":
A celebrated Babylonian amora; born 352; died 427; reestablished the
academy at Sura, and was the first editor of the Babylonian Talmud.
According to a tradition preserved in the academies (Ḳid. 72b), Ashi
was born in the same year that Raba, the great teacher of MaḦuza,
died, and he was the first teacher of any importance in the Babylonian
colleges after Raba's death.
A particularly important element in Ashi's success was the length of
his tenure of office as head of the Sura Academy, which must have
lasted fifty-two years, but which tradition, probably for the sake of
round numbers, has exaggerated into sixty. According to the same
tradition, these sixty years are said to have been so symmetrically
apportioned that each treatise required six months for the study of
its Mishnah and the redaction of the traditional expositions of the
same (Gemara), thus aggregating thirty years for the sixty treatises.
The same process was then repeated for thirty years more, at the end
of which period the work was considered complete.
The artificiality and unreality of this legendary account are made
clear by the facts that the treatises are of different degrees of
length and difficulty, and that a large number of them possess no
Gemara whatever. Probably all that is historical in this statement is
that Ashi actually revised the work twice—a fact that is mentioned in
the Talmud (B. B. 157b). Beyond this, the Talmud itself contains not
the slightest intimation of the activity which Ashi and his school
exercised in this field for more than half a century.