Responding to the original post, but in the form of a total lawyer cop-out: "It depends".
On a course that is boring or a slog to walk/play, I think three hours sounds about right. Whether I get in a full 18 or some portion, I start to lose interest at that point, which is a surefire sign that I have had enough. Given how flat the land is here in Chicago, there are a number of courses where three hours is just right and I wish more of them were routed so that you could easily step off a hole and into the bar earlier than 18. For what it is worth, Beverly (which falls into the bucket I describe below, rather than this) is well routed for just this phenomenon. Seven green and 16 green both end up very close to the clubhouse, giving you an opportunity to bail a little early if you need to.
On a course that is a pleasure to play (see Ira's post about fun) and is well routed for walking, I will gladly play from sun up until sun down. That is part of what I loved about Bandon. We had the first tee time at Pacific Dunes, played Bandon Trails and then ended the day going around the Preserve until we couldn't see our shots land. Other than a stop for lunch, a stop to change our shoes and quick stops at the halfway house, we played, quite literally, all day. Bandon is not alone with courses that fit this description. I could easily see playing all day at any number of courses. Jason Way has a nice metric for these sorts of courses, I think he calls it 108 (holes) in 48 (hours).
So again, it all depends. Golf trips, devoid of distractions make it easier to play all day, but the course and the company impact how long I am willing to play as well.