I think first dates are best set up so that they can last as long as one or both are comfortable and are enjoying each other's company. All of this within reason of course, since I believe it's best for a woman to be able to leave before it's dark outside.
Another or different factor might be how well they know each other before this first date due to the length of time they have been communicating. Then this can change the answer.
In my humble opinion only.
Good question . . . my first F2F last year lasted 12 hours! But only because we had hit it off so well in the communication area.
Did these sites say why?
I like a family restaurant after lunch but before dinner . . . coffee and a croissant (or wine and bread) and a quick get a way when the conversation starts to lag.
Well, I believe that it must be more about personal preference...and not about a set standard, just because someone else "tells" you that this is the way it "should" be. Actually, as a personal suggestion, I would suggest that two people don't set a "time limit" on a lovely evening! Simply go along with the flow, and enjoy the company. Take your time, let the conversation progress naturally, and enjoy getting to know one another better! The best and most meaningful times we share...are those which are allowed to flow naturally in a relaxed atmosphere, and one in which you can both enjoy yourselves and make the most of the time spent together!