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Below is what I found about the subject:
For a 12-year sentence, with full good conduct time (54 days/year), the inmate can reduce the sentence by about 15%.
So maximum time in prison (if no credits, no good behavior) would be the full 12 years (minus any time already served before sentencing, etc.).
Minimum possible time, assuming everything goes ideally (maximum good behavior, all earned credits, etc.), would be roughly:
12 years × 0.85 = 10.2 years
So around 10 years and 2-3 months might be feasible under ideal conditions.
There might also be other small reductions via earned time credits under First Step Act, but those usually affect confinement conditions (halfway house, home confinement) rather than huge reductions of the core prison term. Also, eligibility depends on offense type, behavior, availability of programs, etc.