You definately get higher holding it in. I just don't know what the "ideal" time is. You see, thc will be absorbed into your lungs at some rate, at some point there will be much less thc in the smoke, and therefore you are getting thc deposited at a low enough rate, that the "highness" will be marginal. I don't know how long this takes, so I try to hold it long enough that I'm past that point. 15 seconds works out well for me. I've heard 3 seconds, but that doesn't seem to work nearly as well. It probobly depends largely on the size of the hit you've taken, and the size of your lungs. I've done the no smoke exhale too, it has a name, maybe ghost hit? I can't remember for sure, and I'm too lazy to look it up.