More people makes it take longer. The turns are fast, but once you get 6 people, one round just takes longer.
What's a normal 4 player game like? Or 5 player.
It depends on your victory conditions. We usually play until the first (sometimes second) capital is taken. Otherwise, I'd say a four player game could last a few hours; five or six players could go several hours.
In my experience, two-player games take about an hour, while each additional player probably adds 30 minutes or so. Like anything, its slower the first time, but its pretty easy to get the hang of so by the second or third game, you should be humming along nicely.