I spent most of the day cleaning and cooking because we had guests coming for dinner. Then they only stayed a little over an hour. The last time we had people over it was nearly the same scenario. I can’t help but feel a little disappointed to get so little return on the time and effort invested. (And also find this a surprising thought for an introvert.)
Are my expectations out of line? Are we supposed to ask guests to stay longer when they first hint at leaving soon? What is a typical amount of time to stay when you go to some one’s home for a meal?
first teacher 
I don't know. Who did you have for company? I would have stayed more than an hour.
I wonder if, in this fast paced life we lead, people think they will impose if they stay too long. I like to have something planned, a game or a movie etc. in case we don't just naturally fall into a great conversation or something. (I agree with you.That seems a little rude to 'eat and run'.)
I would think the average stay would be more than an hour. But sometimes if you have kids and the other people don't or vice versa may lead to shorter visits.