Being a trustee is a privilege and a sign of trust from the facility, but it does not automatically translate into good time credit in most county jail systems.
Good time is calculated based on the policies of the specific jurisdiction, and county jails vary significantly in how they handle it. Some counties do award additional credits for trustee status as an incentive for good behavior and reliable work. Others treat it purely as a housing and work classification with no direct impact on the release date calculation.
With three and a half months remaining, the difference a trustee designation might make on the timeline is limited even in counties that do award extra credits for the status. The good time that matters most at this stage is the standard credit already built into his sentence for clean conduct throughout his stay.
The fastest way to get a definitive answer is to have your husband ask his case manager or the jail administrator directly whether trustee status carries any additional good time credit at that specific facility. That question takes a few minutes to ask and produces a clear answer rather than a general estimate.
In the meantime, three and a half months is genuinely short time. The most important thing he can do right now is keep his record completely clean, perform his trustee duties reliably, and avoid any situation that could cost him good time he has already accumulated. This close to the door, protecting what he has built is more valuable than anything else.