Actually, THC is the primary phycoactive in cannabis. [Delta -9 - tetrahydrocannabinol.]

The phycoactive compounds including THC are absrobed almost instantly, so you should inhale, and exhale as you would if you were just breathing normally. 3 -5 seconds tops, any longer than that and as Bobbong said i think, you'll be holding in tar.

People think holding it in gets them higher, but it doesnt. 2 simple words for it.

Oxygen Deprevation, and or Placebo.

But each to their own... if people want to hold it in for stuopid amounts of time then start planning your funeral now.