From Calguns Wiki

Federal Residency Requirements

Permanent

27 CFR 478.11

State of residence. The State in which an individual resides.
 
An individual resides in a State if he or she is present in a State with 
the intention of making a home in that State. 

If an individual is on active duty as a member of the Armed Forces, the
individual's State of residence is the State in which his or her
permanent duty station is located. 

An alien who is legally in the United States shall be considered to be a
resident of a State only if the alien is residing in the State and has
resided in the State for a period of at least 90 days prior to the date
of sale or delivery of a firearm. The following are examples that
illustrate this definition:

Example 1. A maintains a home in State X. A travels to State Y on a
hunting, fishing, business, or other type of trip. A does not become a
resident of State Y by reason of such trip.

Example 2. A is a U.S. citizen and maintains a home in State X and a home
in State Y. A resides in State X except for weekends or the summer months
of the year and in State Y for the weekends or the summer months of the
year. During the time that A actually resides in State X, A is a resident
of State X, and during the time that A actually resides in State Y, A is
a resident of State Y.

Example 3. A, an alien, travels on vacation or on a business trip to
State X. Regardless of the length of time A spends in State X, A does not
have a State of residence in State X. This is because A does not have a
home in State X at which he has resided for at least 90 days.

College Students

ATF FEDERAL FIREARMS REGULATIONS REFERENCE GUIDE p 126

27 CFR 178.11: MEANING OF TERMS

An out-of-State college student may establish residence in a State by
residing and maintaining a home in a college dormitory or in a location
off-campus during the school term.

ATF Rul. 80-21

"State of residence" is defined by regulation in 27 CFR 178.11 as the
State in which an individual regularly resides or maintains a home. The
regulation also provides an example of an individual who maintains a home
in State X and a home in State Y. The individual regularly resides in
State X except for the summer months and in State Y for the summer months
of the year. The regulation states that during the time the individual
actually resides in State X he is a resident of State X, and during the
time he actually resides in State Y he is a resident of State Y.

Applying the above example to out-of-State college students it is held,
that during the time the students actually reside in a college dormitory
or at an off-campus location they are considered residents of the State
where the dormitory or off-campus home is located. During the time
out-of-State college students actually reside in their home State they
are considered residents of their home State.

[ATFB 1980-4 25]

Hat tip to NavyLT at The High Road.