USA Tourist Visa: Travel to Mexico and Canada
USCIS now allows travel to Canada and Mexico for a period of one month if you have a B1/B2 Visa. However upon re-entry in the USA you will not be subjected to POE and the same visa duration that was initially issued will be applicable.
Example: If you have a USA Tourist or Business or B1/B2 Visa, and at the point of entry you are permitted 6 months of stay in the USA, you can travel to Mexico or Canada for a period of one month. So if you leave in the 4th month and return on the 5th month to USA, you will still have one month of stay in USA. No new period will be allotted during the Re-Entry in USA.
Source: CBP Website