You can estimate that they will serve 85% of their sentence. If you want a more definite date, contact the Clerk of the Court where they were sentenced.
https://www.inmateaid.com/ask-the-inmate/how-do-i-find-out-the-approximate-date-of-an-inmates-release-without-contacting-the-inmate#answer
Accepted Answer Date Created: January 09,2021