Sometimes inmates get sentenced to life with a possibility of parole. 15 years could pass and parole could be granted or even 20 to 30 years. Or sentenced 30 years to life. Which means they can be released after spending 30 years in jails 1st and nothing less it could go over but at least 30 in jail. Or just flat out LIFE With no parole so you will basically rot in jail.
Depends on what state the person is sentenced in. Some states vary from 25-40 years. The keywords are from the judge. "I sentence you to life in the department of corrections," can be the 25-40 year thing! "I sentence you to the department of corrections for the REST of your natural life!" You come out in a box!