It will be a two-hour series finale.
The first hour will conclude the 2-part "Terra Prime" episode.
The second hour is the finale episode, "These Are the Voyages..." written by Berman & Braga. The story will take place six years in the future. Archer & Co. returns to Earth for the decommissioning of NX-01; and bear witness to the signing of the Federation Charter.
Whatever your feelings about this show, it will be missed.