How long do you serve on a 40 yr sentence with 30 yrs in already?

Depending on the prison system that your inmate is in, the typical offender serves 85% of the time imposed. This 15% represents "good time" credited when the sentence begins. Assuming that the inmate has all of their good time intact, 85% of 480 months is 408 months of which 360 have been served. That leaves 48 months or exactly four years left to be served.