* Hours required for graduation: This is the most probable meaning. Senior year is the final year of college, and "senior college hours" might be used informally to refer to the number of credit hours a student needs to complete *during* their senior year to graduate. This number varies significantly by college, major, and individual student circumstances.
* Hours completed by a senior: Someone might use this phrase to describe the total number of credit hours a senior student has already accumulated. Again, this is not a standardized term.
* Hours in senior-level courses: This could mean the number of credit hours a student is taking in courses specifically designated for senior-level students (e.g., courses numbered 3000 or 4000 in many US colleges).
To understand the meaning, you'd need to consider the context in which the phrase was used. If you encountered this phrase, it's best to clarify its meaning with the person who used it.