Limoncello takes some time but it's definitely worth it! Use an alcohol content calculator to dilute the limoncello to your desired proof. The longer this rests, the mellower it gets.
You might need 22 lemons, depending on size. Also excellent when using grapefruit or orange zest.
You may not use all of the simple syrup - typically I use 10 cups simple syrup to bring the ABV to 40 to 50 percent.