Upvote:3
A US visa is only for entry and the visa's expiration date just means the last day on which you can use the visa to enter the US. It has nothing to do with how long you can stay in the US. How long you can stay in the US during that stay is determined by your status, as given on your I-94 at entry and subject to regulations regarding your status. An expired visa doesn't mean you can't stay in the US (you can if you are in status), and conversely a valid visa doesn't mean you can stay in the US (you can't if you are out of status).
To determine how long you will be admitted for, you look at the regulations for your status. If you have a B2 visitor visa, you are usually admitted for 6 months (although it can be shorter if your passport expires sooner, or in some cases if the officer feels like you've been to the US too frequently). So for example, a visitor on B2 visitor visa can enter on the day the visa expires, and usually be admitted for 6 months, most of which will be after the visa's expiration.