Thomas,
An interesting question, and one that has a couple of different answers, at least for me. I'll assume we're talking a four ball here, and not solo rounds, and walking.
For casual play, I think 3:45 is fine, which is typical for my regular group, and I'm ok up until about 4:15. Anything longer than that starts to annoy me, but I also don't come to the golf course to see how quickly I can get home. Of course, I'm retired, and so I'm not trying to beat darkness, or get home to take care of the kids, or any of that stuff anymore. I come to the course to be outside, to enjoy my buddies, and to try to play good golf; finishing in under 3 hours seems counterproductive to all of that. The unstated ethic of "I want to go to the golf course to see how quickly I can leave the golf course." just doesn't click with me.
For stroke play tournament golf, the equation changes significantly, and 4:30 is more realistic. There are a lot of guys on this site that will scream about that pace of play, but it just isn't possible to play competitive tournament stroke play golf much faster than that.