B-1 and B-2 visitors normally get six months each time they enter, but that should be indicated explicitly by a date on the entry stamp. This is the meaning of the word "until" on the stamp: that is the place where they are supposed to write or stamp the date by which the traveler must leave the United States.
If they fail to put a date on the passport stamp, you can look up the record in the I-94 database at i94.cbp.dhs.gov.
In some circumstances, the immigration inspector may grant a shorter period of admission. It is of course also possible to be refused admission, but something would have to be obviously amiss for that to happen.