By Kathryn Berck
Allowable length of stay is given individually, on arrival in the US, by the CBP officer who greets the traveler. It has nothing/nothing to do with the validity dates of the visa itself.
For a B1/B2 visa holder they will usually give six months even if the visitor doesn’t intend to stay that long. That just simplifies the work load if, for some unexpected reason (illness, accident, family drama), a person who meant to stay for only a few weeks finds they have to stay longer for a legitimate reason. It saves the person – and themselves – a lot of work processing extension of stay requests.