The Supreme Court of the United States struck down one-year residency requirements to vote in Dunn v. Blumstein 405 U.S. 330 (1972).<23> The Court ruled that limits on voter registration of up to 30 to 50 days prior to an election were permissible for logistical reasons, but that residency requirements in excess of that violated equal protection as granted under the Fourteenth Amendment according to strict scrutiny.
this site has a list of states and what their residency requirements are..
http://www.infoplease.com/ipa/A0781452.htmlResidency Requirements for Voting
The Supreme Court decision of March 21, 1972, declared lengthy requirements for voting in state and local elections unconstitutional and suggested that 30 days was an ample period. Most of the states have changed or eliminated their durational residency requirements to comply with the ruling, as shown. Note, for all states, in order to register to vote, an applicant must be a U.S. citizen, a legal resident of the state, and 18 years old on or before election day. Additionally, most states do not permit an individual to vote if he or she is a convicted felon currently serving time in prison or has been declared mentally incompetent by a court of law.
this site has an interactive map that sheds some clarity..
http://www.brennancenter.org/studentvotingStudent Voting Rights
The Twenty-Sixth Amendment gives 18 to 21 year olds a voice in our democracy. Students who leave home to attend college should have the right to choose where they vote. In most states it is possible for students to make this choice, but the laws governing voting eligibility can be tricky, confusing, or downright restrictive. Students can be unfairly targeted by election officials or partisan challengers and often have trouble establishing residency where they live and attend school.