https://www.marxists.org/history/ussr/events/revolution/documents/1917/10/29.htm

In the Soviet Union, the eight-hour day was introduced four days after the October Revolution, by a Decree of the Soviet government in 1917

https://www.marxists.org/history/ussr/government/1928/sufds/ch17.htm#%3A~%3Atext=On+the+other+hand+the%2Cworking+day+averaged+7.5+hours.

The labor efficiency has risen through better industrial processes and better machinery. Output per worker in the Soviet Union is still low by Western European standards, but it has been showing a healthy rate of advance. The advance has been aided by a campaign against absenteeism. In 1913 the days of actual work per worker in industry were 257. In the fiscal year 1921-22 they had fallen to 219.5. In 1926-27 they were 262.1.

On the other hand the length of the normal working day, which was 10 hours before the war, was reduced to 8 hours at the beginning of the Soviet régime, and for dangerous occupations to 6 hours. During 1926-27 the working day averaged 7.5 hours.

The Council of People’s Commissars, on the occasion of the tenth anniversary of the Soviet State, in November, 1927, decreed the gradual introduction of the 7-hour day in industry. This is being put in force first in the textile industry.

From the 1928 release of Soviet Union Information Bureau report on labor.