Assumption: You're from a visa exempt country (see
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9.pdf).
1. You are entitled to two visa exempts by land each calendar year. Each entry is good for 30 days, which can be extended by another 30 days.
2. You are entitled to "unlimited" (as defined by the immigration officer on duty) by air. Each entry is good for 30 days, which can be extended by another 30 days.
3. There is no "180 day" limit, or a maximum of X entries by air. HOWEVER, the immigration officer at the airport will prevent you from entering if s/he thinks you're abusing the system and living here on visa exempts.