For the current fourth quarter, if the Dow drops 1,100 points before 2 p.m., trading stops for an hour. If such a drop happens between 2 p.m. and 2:30 p.m., trading halts for a half hour. After 2:30 p.m., the 1,100-point threshold expires.
There is also a 2,200-point mark. If the Dow falls by that much before 1 p.m., trading stops for two hours. Between 1 p.m. and 2 p.m., a 2,200-drop causes an hour halt. After 2 p.m., trading ends.