I really don't think holding it in for more than ~5 seconds is really gonna make much of a difference. It may get you a tttiny bit higher, but not enough that you are going to notice a difference. Honestly, I've held it in for 2 seconds and got just as high as I would have had I held it in for 10 seconds. Again, I am basing this on my own experience, of course. But I really don't think it makes a noticeable difference. As long as you INHALE! I've met people who didn't even know what that meant. Make sure you're inhaling, otherwise you are wasting a lot of weed.
I've learned from a lot of my friends on stonerradio.com that it's unnecessary to hold in the smoke for that long. They hit the bong/pipe/joint/whatever breathe it in and then let it go instantly. They say that when you hold it in you're just depriving yourself of oxygen, and that's a bad thing.
technically it shouldn't even take longer than 5-10 seconds to ghost a hit of you take ones that are small enough
I hold it in for 3-6 seconds. I know nothing of how long one should hold it in for optimal effect (it seems like no one does) but I safe it, because I do know that if I don't hold it in for at least a couple of secs, I won't get as high.