The B1/B2 visas are short term visitor visas intended for business and pleasure. In other words, the B1/B2 visas are tourist visas. When a B1/B2 visa holder enters the United States, they are given an I-94 document stating the amount of time they are allowed to remain in the United States. The expiration date of the stay is left completely up to the interviewing customs officer; in some cases a visa holder may be granted one month, while another six months. Failure to leave by the stamped exit date results in cancellation of the visa and impedes future re-entry.
There are two types of B1/B2 visas: multiple entry and single entry. Multiple entry visas are marked with an "M" and allow the holder to enter the United States for an unlimited number of separate visits. Single entry visas marked with an "S" are valid for only one entry into the United States for a stay lasting between 90 and 180 days. Multiple entry visas are valid for multiple entries for a period between five to 10 years, however, each stay is also limited to between 90 and 180 days. If a single entry B1/B2 visa holder wishes to re-enter the United States, they must apply for a new visa through the U.S. Consulate General.
Travel to Canada and Mexico
The only countries you can travel through and re-enter the United States from and have it still be considered one continual stay are Canada and Mexico. B1/B2 visas allow the visa holder to enter and remain in either country for up to 30 days as long as the visa holder re-enters the United States before the expiration date of their I-94.
Any time B1/B2 visa holders leave the United States for any destination other than Mexico or Canada, they must surrender their I-94 to the customs officer when departing the country, even if it has not yet expired. Re-entering the country afterward is treated as an entirely new "entry," and the multiple entry visa holder is given a new I-94 upon each new re-entry. If the new entry is within a close time frame of the last entry, the inspecting customs officer may inspect travel-and-exit plans more thoroughly and --- if they have reason for suspicion --- deny entry.
Interval Between Visits
There is no set law that states how many days a multiple entry B1/B2 visa holder must remain outside of the United States before re-entering. Like most situations concerning immigration, the amount of time needed between entries varies by individual and the circumstances surrounding the stay. Persons travelling to the United States for short business trips of one to two weeks can often enter the country between 10 and 15 times a year, as their short stays prove their intention to not remain in the country as a de facto resident. If a B1/B2 visa holder remains in the United States for periods of three to six months at a time, it is generally best to wait three to six months after leaving before attempting to re-enter, or the customs officer may find the frequent long-stays suspicious. It is the right of the inspecting officer at the U.S. port of entry to deny re-entry on the grounds of any type of suspicion.