With a foursome of mediocre golfers we usually finish in a little over 4 hours. Mediocre twosome a little under 4 hours. That's with a short stop for lunch at the turn and includes a bit of waiting at the tee boxes for the group ahead of me.
18 holes, me and a buddy, ususally around 4 hrs. but we let damn near everybody play through, we're just not in that big of a hurry. Hell thats just more time away from the wife!!