I have never noticed any better results by holding smoke in for a longer period of time. If you think about it, the lungs are lined with capilaries that absorb everything via osmosis very quickly. Just a sec longer than a cigerette is fine. Any longer and the only extra effect you are going to get is a crappy cough when you exhale.

When I was young I bought into the "hold it in" theory. All it did for me was make me cough up awful shit every time I took a hot shower.