Levels of Measurement
Nominal: the data can only be categorized. For example: city of birth, gender, ethnicity, car brands, etc.
Ordinal: the data can be categorized and ranked. For example: Top 5 Olympic medallists, Language ability (e.g., beginner, intermediate, fluent), etc.
Interval: the data can be categorized, ranked, and evenly spaced, but there is no true zero point. Examples: Test scores (e.g., IQ or exams), Personality inventories, Temperature in Fahrenheit of Celsius, etc.
Ratio: the data can be categorized, ranked, evenly spaced, and has a natural zero. The lack of the property is always measured as zero. Examples: Height, Age, weight, temperature in Kelvin.
The variable measured is the time served, or the time period between when the dependent is arrested and when they are convicted. It can be categorized, ranked, and evenly spaced.
The question is: Is there a natural zero? Is it possible to have zero time served. If the answer is yes, then this is a scale of measurement type ratio. If not, then its an interval. I personally think it's an interval because once the dependent is arrested, its time starts to run and it cannot be zero, even if they get released the next hour.