U don't get a damage buff for the whole round...
It lasts the normal time that ss would last for (6secs or 8 for reg and 10 or 12 secs for enh) don't remember exact length offhand.
The timer is paused in-between rounds so you get a boost for the normal time the start of the next round.
This was a known issue that left alone as a gimmick for Shang players. Why? Because I felt like it.