Institutions widely use student evaluations to assess the faculty's teaching performance, but underlying trends and biases can influence their interpretation. Using data from Rate My Professors, we conduct the largest and most recent quantitative data analysis to study questions related to the evaluation criteria that students have when they review the performance of their male and female professors. Our analysis spans data from two decades (1999-2019), thus taking into account recent changes on the website and in the perception of students, and demonstrates interesting insights related to how students perceive the teaching style and personality traits of their male and female professors. We also present the first analysis that investigates how gender bias evolves over time and changes over space. We believe that our results are interesting from a sociological viewpoint, as they investigate the role of gender in higher education by disclosing how students perceive and evaluate professors of different genders. In addition, we believe that our findings can be useful to educational institutions when considering possible biases that exist in the evaluations of their faculty.