Table 3, 2 crew, up to 13999 ft, 100%, gives 22 l/min
3150 l / 22 l/min = 143 min = 2:23 hours.
At higher pressure altitudes, the air is less dense, so 100% requires less oxygen, and endurance is longer.
The Table 3 note suggests that at 8000 ft the rate should be 26 l/min.