What attitude should a speaker have to going over time? From John Baez's Advice for a Young Scientist:


  Your talks should be clear, concise, fun, exciting, and never ever run over time. For each extra minute your talk runs over, 10% more of the audience will decide you are a jerk and start fantasizing about you falling down a trap door.


After 11 minutes, 110% of your audience will start hating you.

That makes perfect sense, because that includes the people waiting for the room to open up for the next talk to begin.

But some people at my college like to take the opposite view. One lecturer never releases his class until he's run at least five minutes overtime. In the mandatory presentation class, the teacher maintained that going five minutes over the twenty minute talk period time was ok, but when one student went one minute undertime, she insisted that he repeat the assignment.

My question is: how should the speaker treat overtime?
Speaking for longer than the allotted time is unprofessional, and it can be very damaging and offensive in some circumstances.  For example, if a conference schedules talks back to back, then it's not acceptable for one speaker to try to use part of the next speaker's time.

It may not be quite as bad in other cases, but it's still disrespectful to the audience.  It tells the audience that the speaker believes his/her final words are more important than whatever else the audience might need to do, and it forces anyone with other obligations to risk drawing attention by leaving before the end of the talk.

Classes are something of a special case for two reasons.  One is that the professor has a certain degree of power over students in the classroom, unlike a typical seminar speaker; another is that it's easy for professors to rationalize that they are going over time for their students' own good.  Going over time is still problematic, but some people don't feel bad about it.

As for how to handle it, this problem should never even come up if the speaker is on top of things.  It's important to keep track of the remaining time and adjust the presentation to omit details as needed to finish on time.  If you screw this up, then you are out of luck if anyone is scheduled to use the room after you.  Otherwise, you could apologize and offer an extremely brief summary of your remaining points.  (I.e., if you suddenly discover that your time is up, that's when you should switch to the 30 or 60 second summary of your conclusions, rather than continuing with the seven minutes remaining in your talk as planned.)