To be technical for a moment - it is entirely possible to have an expired Visa and and still be in the USA legally. The I-94 given at the POE determines the length of stay; the Visa only determines the time period that a person has to present the Visa to the POE and apply for entry.

The other factor is the validity of the presenters passport - normally it is preferred to have a PP that expires 6 months after the end of the proposed visit.

I know this for fact because I have had a valid I-94 well after the Visa had expired - At the time of entry (with the valid Visa), I asked CBP agent if I should get a new Visa but he simply gave me long enough to travel back & forth from Vancouver, BC while I had family visiting the Northwest & touring. Saved me a bunch of time, and I got my new visa after the family had finally gone home!^^