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The usual limit is six months per visit. This is codified at 8 CFR 214.2(b)(2):
Minimum six month admissions. Any B-2 visitor who is found otherwise admissible and is issued a Form I-94 (see Β§ 1.4), will be admitted for a minimum period of six months, regardless of whether less time is requested, provided, that any required passport is valid as specified in section 212(a)(26) of the Act. Exceptions to the minimum six month admission may be made only in individual cases upon the specific approval of the district director for good cause.