If you declare it, chances are you won't be charged with any duty. If you are, it's something small like $0.01 per % of alcohol per liter (don't remember the exact numbers but that's the order of magnitude). So a liter of vodka would be $0.40.

Be aware though, if your state has restrictions on the amount of alcohol one can carry/import, that will be more important than anything the fed laws say about it. I don't know how vigorously the feds enforce state laws, but it probably varies from airport to airport.