>10 percent in any 12-hour period: projected 1 hour halt
>20 percent in any 12-hour period: projected 2 hour halt
>30 percent in any 12-hour period: projected close until noon the following day
The bitcoin market, due to its immaturity, will inherently be volatile. If there were circuit breakers in place, in would have to be far in excess of 10 percent. Perhaps something along the lines of 20 percent in a 5 minute period, or 50 percent in a 30 minute period.
More importantly, if such circuit breakers were implemented, they must be publicly stated in the exchange's rules, and participants can evaluate the rules in place by the various exchanges in making a decision on which exchange to use.