In a blender, mix two 12-oz. bags frozen dragon fruit, slightly defrosted, or 3 fresh dragon fruit with flesh scooped out, 1/4 cup granulated sugar, 1/2 cup water, and the juice of 1 lime (about 2 Tbs.) until smooth. Pour mixture into a 3-quart baking dish. Freeze, covered with aluminum foil, for 2 hours, stirring and scraping mixture from sides of dish every 30 minutes. Freeze 6 more hours, without stirring, until firm. Serve immediately or keep frozen for up to 2 weeks. Scrape with a fork immediately before serving.