The AAAS Science & Technology Policy Fellowships are one-year opportunities. Some of the fellowship assignments in federal agencies may be renewable for a second year, at the mutual agreement of the hosting office, the Fellow, and AAAS. Congressional Fellowships are available for one year only.
For most Fellows, the annual stipend and benefits are administered by AAAS. Some Fellows are hired directly by hosting offices under temporary federal employee status and receive their annual stipend and benefits directly from their office.
A nice opportunity for someone heading out on sabbatical: