Most that I've been to have been about 3 hours. The last one I went to for a friend was almost 4 hours, and that seemed to drag on and on. She didn't start opening gifts until 3 hours into it.
I went to a shower once where it was easily 3 hours before the gift opening started - and I left! I had blocked out 3 hours and couldn't stay.
They spent the 1st 3 hours w/ eating, mingling, and at LEAST 5 games, and it might have been 6.
I think it was this shower that actually fully pushed me into being a "I hate shower games" person. It was too much, and they went on for too long. Insane time-suck. I was so annoyed.