This recipe comes direct from Hadley at Gourmandise Desserts in Los Angeles. Her original recipe states it will last 1 to 2 weeks in the refrigerator. I got about a month and a half life out of it.
3 cups sugar
2 seedless oranges, thinly sliced
3 cups water
Place sugar and 3 cups water in a medium saucepan. Bring to a boil over medium-high heat, then reduce heat to medium once the sugar has dissolved. Add oranges. Simmer until rinds become very soft and syrup begins to foam, about one hour. Cool to room temperature. Store in an airtight container in the refrigerator.