Final answer:
The provided table does not indicate the minimum duration for an open enrollment period. This information generally comes from regulators like CMS or state laws, but typically the period is around 30 days.
Step-by-step explanation:
The question asks about the minimum duration for an open enrollment period that an HMO must offer at least once every 12 months after the initial enrollment period. The specific duration requested is not provided in the provided information. Table 2.91 displays enrollment frequency data by the size of the group but does not indicate the minimum number of days for an open enrollment period. To provide a concrete answer, one would generally refer to the guidelines set by the Centers for Medicare & Medicaid Services (CMS) or specific state regulations, as these periods can vary depending on the regulations that govern the HMO. However, it's common for HMOs to offer an annual open enrollment period that typically lasts around 30 days, though this can vary.