Background: There have been repeated calls for more widespread use of inquiry-oriented science instruction in K-12 education. At the same time, questions have been raised regarding whether the amount of time in school schedules for science instruction is sufficient to support inquiry-based teaching. This study investigates the relationship between the time available for science instruction and the extent to which eighth-grade science teachers used inquiry-oriented instructional practices in their teaching. Research Questions: We consider two research questions: (1) To what extent is teachers' use of inquiry-oriented instructional practices related to the time available for science instruction during the school week? (2) To what extent do differences in teachers' professional training to teach science impact the relationship between instructional time and science teachers' use of inquiry-based instructional practices? Research Design: We use data from the 2011 NAEP Grade Eight Science Assessment and multilevel linear modeling to analyze the relationship between instructional time and science teacher practices. Our analyses include approximately 11,520 eighth-grade teachers in 6,850 public schools. Findings: The extent to which science teachers used inquiry-oriented instructional practices in their teaching was related to the amount of time available for science instruction. Teachers with 5 or more hours per week were more likely to use inquiry-oriented instructional practices, and the extent to which teachers engaged in reform-oriented science instruction increased with more time for instruction. The relationship between instructional time and teacher practice was largely independent of teacher qualifications, suggesting that instructional time impacts teacher instructional practice regardless of teachers' educational background, training, or science teaching experience. Conclusions: Identifying optimal allocations of instructional time is a relevant consideration in efforts to promote the types of reform-oriented science teaching called for by the National Research Council's Framework for K-12 Science Education, the Next Generation Science Standards (NGSS), and other national and state science assessments. Although investments in teacher training and professional development are also necessary and important investments, with insufficient time, even the most qualified or best trained science teachers may struggle to use inquiry-based instructional approaches.