THC is absorbed in the lungs within a few seconds. There is no need to hold in the hit any longer than 5 seconds. If you've ever heard that you need to hold the hit in as long as you can, its false. Back in the 60's smokers believed you need to hold the hit to get high, but due to recent scientific studies, it has been debunked.

If you hold the smoke in your lungs any longer than 5 seconds, your only holding in the tars and toxins, and not getting any higher.