How long do I have to be physically present in the US in order to qualify for citizenship?

To qualify for citizenship, an applicant must not only be a U.S. permanent resident for the required time period, but must also demonstrate that he has maintained continuous U.S. residency for a certain period of time. For those applying based on marriage to a U.S. citizen, the requirement is 3 years of continuous residence in the U.S.  Such applicants must also prove that they were physically present in the U.S. for at least half of that time (18 months).

For those applicants not applying based on marriage to a U.S. citizen, 5 years of continuous residence is required. Such applicants must also be physically present in the U.S. for at least half that time (30 months). 

For all applicants, absences of six months or less from the U.S. do not break an applicant's continuous residence.  However, absences that are six months or more may result in a break of the continuous residence. Once broken, an applicant must re-accrue continuous residence.

In some instances, an applicant who must take an extended trip outside of the U.S. may file an application to preserve residence for naturalization. If approved, this permits a permanent resident to meet the continuous residence requirement even though he is not physically present in the U.S. for a prolonged period as required.