I believe it was six days a week until the Homestead strike was broken. Then Carnegie upped it to seven days a week. A few details are here:
https://www.reference.com/business-finance/did-andrew-carnegie-treat-his-workers-de36d945a374a10f
I think you are right. The 12 hour shift remained standard until after the 1919 steel trike collapsed. In 1920 the giant steel companies (US Steel, Bethlehem) instituted the eight hour shift and the 24/7 operational mode. Wages remained stationary so de fact significant wage hike was put in place. “Little Steel’ followed soon after. This was also the start of the complex staggered shift and while unions other than company ones were banned workers could bid on the shift/days they wanted with seniority deciding who got what. These changes were due to a big steel management study, or a series of studies, to position the industry to be flexible and agile to meet coming demand changes for specialty steels.