Never go over your alloted time when you're giving a talk.
This is particularly an issue for people giving presentations at conferences, where your talk is just one small part of a much larger parade of presentations. If you go too long, you make everyone else late for the rest of the day.
If there are multiple tracks of presentations, the imperative to stay on time gets even greater. People will often move from one room to another, popping in to see one talk and then leaving to see another talk. If the schedule goes out of whack, you do a great disservice to the audience.
My general rule of thumb is aim for your talk to be about 80% of alloted time. Given a 15 minute talk? Aim for 12. Got a 50 minute talk? Aim for 40. That way, if you are delayed a little, you can still finish on time.
And how do you know how long your talk is? Rehearse, rehearse, rehearse.
Nobody ever criticizes a talk for being too short. But having a talk that goes on too long -- not knowing when to shut up -- is really the height of rudeness.