Use a vegetable peeler or zester to remove the peels from all of the lemons, avoiding peeling the bitter white pith. Place the lemon peels in a large jar and cover with alcohol. Allow the peels to steep for four weeks somewhere dark, like a closet or cabinet.
Strain the infused vodka into a large bowl and discard the lemon peels. Add one cup of simple syrup to the vodka and stir. Taste for sweetness, adding more simple syrup as desired. Use a funnel to bottle the limoncello and return to a dark area for two more weeks for the flavor to mellow. Chill thoroughly in the freezer before serving.