Background: Office hours are one of the most ubiquitous resources for students in STEM courses. However, there has been only limited work examining what students think of STEM course office hours, and we are not aware of any past work that has examined (1) how students perceive STEM course office hours upon entering college, and (2) how these perceptions change over time and the factors that shape these perceptions and expectations of what will occur in office hours. Here, we utilize a longitudinal series of surveys to capture first-year students' perceptions and expectancies for office hours at the beginning of college, after one semester, and at the end of their first year, drawing upon expectancy-value theory to situate our results. Results: We identify that most new first-year students enter college with moderate or high self-reported familiarity with office hours, and that the level of familiarity increases throughout the year. New first-year students hold a variety of conceptions about office hours, some correct (e.g., associating office hours as a space for getting help on course content) and some likely incorrect (e.g., office hours as an independent study hall), that likely shape students' expectancies, perceived benefits, and costs of attending office hours. We also find that most new first-year students hold positive attitudes towards STEM course office hours, and that some students' perceptions are likely shaped by some secondary schools implementing office hours during the COVID-19 pandemic. Finally, we determine that most students who attend office hours report them as helpful and increasing their self-efficacy in the course, with students providing a variety of reasons for how office hours impacted their self-confidence (e.g., through increased content knowledge, better relationship with the instructor, greater metacognitive reflection.). Conclusions: Our work provides the first examination of how new first-year students perceive STEM course office hours, and the first longitudinal study that tracks their changes in perception over time. We highlight implications and recommendations for STEM course instructors to increase their office hours engagement throughout, drawing upon our results and expectancy-value theory.