You don't need to. Most THC is absorbed by the lungs immediately. After three seconds, THC absorption is too minimal to make a difference. The harm to your lungs, however, is not.

There's a graph floating around on this site from a NORML/MAPS test that shows this. I don't have it on my pc, but I'll look for it if you want.
graph Reviewed by graph on . How long do you hold your hits in? I hold my hits in for 5-7 seconds. i hate seeing people exhale a huge white cloud of wasted smoke. Rating: 5