Commercial designs don't need to be overhauled, don't suffer from failures, and thus don't need much repair supplies. Also morale doesn't matter except for geo survey on commercial designs, so they need a minimum deployment time of 3 months.
Military designs suffer from failures, annual failure rate depends how often. Repair supplies are used to fix those failures as well as battle damage. Overhaul is necessary if the clock goes over the maintenance lifetime of the ship, as failures will accumulate. Deployment time determines how long a ship can be out before morale begins to drop, which reduces efficiency of the crew.