If you have entered the US on a visa and do not have a green card, you have finite amount of time during which you can stay in the US. If you want to stay for longer than this amount of time, you will need to apply for an extension of stay. If you want to extend your stay, it is important to apply for an extension of stay rather than simply overstay your visa. If you stay longer than you are authorized to remain, you will be in violation of your visa and you will have a hard time being re-admitted to the US on subsequent visits.
One thing you will need to determine as you try to extend your stay is the duration of your stay. This can be confusing, as the date on your visa and the date on your passport may be very different. For example, when you apply for a visa, your visa will have an expiration date. However, when you enter the US, a Customs and Border Protection (CBP) will evaluate your request for admission and will place an Arrival-Departure Record (Form I-94) or Form I-94W in your passport. The official will then add a date or “duration of status (D/S)” on the form. If you have D/S on your form, you may be able to remain in the US for the duration of your visa. If you have a date on your Form I-94 or Form I-94W, you must leave the US by that date, even if the date is well before your visa expires. If your visa allows multiple entries into the US, you can request entry into the US again. If your visa does not, you will need to apply for a new visa before trying to enter the US again.
If you are already in the US on a non-immigrant visa and your date of departure is approaching but you wish to extend your stay, you can apply for an extension of stay by filing the Form I-539 (Application to Extend/Change Nonimmigrant Status) with the USCIS. You must do this well in advance of the date your stay expires. The USCIS recommends that applicants apply at least 45 days before this date.
You can only apply for an extension of stay if your nonimmigrant visa status stays valid beyond the date your stay expires and you can only apply if you have done nothing to violate the terms of your stay while in the US. If your visa or passport will expire during your extension of stay or if you have committed any crime while in the US, you will typically not be granted an extended stay. In addition, some non-immigrant visa holders are not eligible for an extended stay. This includes D visa holders, C visa holders, K visa holders, S visa holders, and those admitted to the US through the Visa Waiver Program or those admitted as in transit through the US with no visa.