I have a B1/B2 visa valid from 15 July 2015 to 15 March 2016. The way I've understood it from the embassy personnel, is that when I enter the country with this visa I'm allowed to stay for 6 months. However, if I for example go home (to the EU) over Christmas and then return to the US before 15 March, will I start a new 6 month period? Intuition tells me this can't be the case... But it'd be useful if it was true.
The decision whether and for how long is made on arrival:
At the port-of-entry, a U.S. immigration officer of the Department of Homeland Security (DHS) decides whether to allow you to enter and how long you can stay for any particular visit, as part of the Admission process. Only the U.S. immigration officer has the authority to permit you to enter the United States.
Six months is the normal maximum but this may be extended Up to 6 months; maximum total amount of time permitted in B-1 status on any one trip is generally 1 year.
Leaving USA terminates the validity of the visa, where that is single entry, or utilises one of the allowed entries where multiple entry. Returning from the EU (after Christmas) is a further entry, the duration of which is again up to the immigration officer – and might be six months.