This is the same period of time as the 42 months (at 30 days per month) when the Gentiles trample the holy city underfoot (Revelation 11:2). The reference to days instead of months in relation to the witnesses points to the intensity of their work, specifically that they witness/prophesy day after day without letting up.
3 And I will grant authority to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth.”