You should feel the effects within 20 minutes. Radiohead's Kid A and Amnesiac are perfect for this experience, as is the music of Sigur Ros. I would caution the kids at home against listening to any Iron Maiden while experiencing the pleasure of rectally-induced hydrocodone. Even the early, pre-Bruce Dickinson stuff. Also stay away from Pantera.
Just make sure you know what you're doing before you give yourself an enema. It is possible to injure yourself if you do it wrong, believe you me! And let me tell you, constantly having to make up stories to tell the ER people sucks. My advice is to think of your rectum as Carnegie Hall and practice, practice, practice!
(Note to self: remember to ask publisher about getting this one syndicated. Maybe Commentary or Tikkun?)