95% of the THC is absorbed almost instantly, as soon as the smoke hits your lungs. So, no, you won't get higher from holding in your hits for seconds upon seconds. You may get a little buzz from the lack of oxygen to your brain...but that's not a good thing. Also, it's been shown that the longer you hold your hits in, the more damaging it is to your lungs and the more it irritates your throat.

So yeah. I'd advise against holding it in for too long. I usually savor it for a second, then let it out slowly.