Most Professors are in tenure track positions. This means that a new PhD commonly starts with a college/university as an Assistant Professor. After approximately 6-8 years (depending on the institution), they are eligible to apply for tenure.
After tenure, the next step is Associate Professor. Many institutions require a professor to be an “associate” for at least five years after achieving the rank.
After approximately 10 years, a professor commonly applies for the rank of full professor.