The options are more limited than most people expect. For the vast majority of state inmates, programming does not directly reduce the release date. Good time is calculated upfront and built into the first projected release date the inmate sees. Completing programs does not add more time off, though it can prevent the loss of good time credits already granted.
The major exception is RDAP, the Residential Drug Abuse Program, which is available only in the federal Bureau of Prisons system. Completing RDAP can earn an inmate up to 12 months off their sentence. It is one of the most significant sentence reduction tools available in the federal system, and it makes a real difference on longer sentences.
The catch is that RDAP eligibility is not guaranteed and has to be planned for before sentencing. The pre-sentence report needs to include the right language to establish a documented substance abuse history, which is required for RDAP qualification. Ideally, this is addressed by the defense attorney before the PSR is finalized. If your person is still in the pre-sentencing phase, this is worth discussing with their attorney immediately.
For federal inmates who are already in and have the qualifying history, getting on the RDAP waitlist as early as possible is the most important step. These programs fill up and placement can take time.