How long can I stay in the US on a B-1/B-2 visitor visa, and can I extend it?

Your visa stamp only allows you to travel to a US port of entry and ask to be admitted. The Customs and Border Protection officer at the border decides your actual authorized stay and records it on your Form I-94. For visitors this is commonly up to six months, though business visitors may be admitted for the time needed for their activities.

If you need to remain longer than the date on your I-94 without leaving the country, you must file Form I-539, Application to Extend/Change Nonimmigrant Status, with USCIS and provide supporting documents. USCIS recommends applying at least 45 days before your authorized stay expires. Always check the I-94 date rather than the visa expiry, because the two are different things.