You're required to file taxes from a state if you've earned income there. You're also required to file taxes based on where you live. Residency determines what forms you file if the state has different forms for residents and nonresidents or full-year or part-year residents. States vary when it comes to how they classify part-year or full-year residents. Some states consider you a full-time resident if you lived there for at least 183 days. Consider consulting a tax professional for assistance if you need to file taxes in multiple states.