This article is from the FAQ, by with numerous contributions by others.
Period of stay regulation:
[ 8 C.F.R. 214.2(h)(13)(i)(13)Admission(i)General(A)
A beneficiary shall be admitted to the United States for the validity period
of the petition, plus a period of up to 10 days before the validity period
begins and 10 days after the validity period ends. The beneficiary may not
work except during the validity period of the petition.
As a matter of practice, INS has been questioning even
one day's time out of status.
Additionally, they can ask that the employer pay for their one way
air ticket to the country of residence.