Going overtime in a presentation is a common failure in scientific presentations. However practicing your talk should prevent this from happening.
The scene is unfortunately quite familiar. A professor or student has 90/60/25 minutes for his or her presentation and it goes overtime. While the speaker keeps talking, the students need to run to another lecture, the next speaker is annoyed, or the audience wants to go to the break. Although we know this shouldn't happen, we see it often at conferences and lecture halls.
I thought it happened because people did not practice enough. But a friend of mine, who also happens to be involved in academia, told me two weeks ago that is not always the case. She said professors and students go overtime to show that they have more say, that it shows their passion for the topic. At first I was skeptical, but I started thinking about some of the talks I have heard/ and delivered in the past, and realized it is also true. People overtime to show they could say more. I guess some students overtime thinking the more time, the better the grade /the more impressed their professors will be, but I would bet in some cases those 5 or 10 more minutes actually hurt their credibility. Maybe your professor thinks you could have said in less time, or that those details were not relevant.
In any case, speakers who overtime fail in two major fronts of public speaking: Practice (or the lack of) and thinking their talk is about them (when in fact is about the audience). I have seen people preparing their presentation visuals the day and/or night before their talk. Let's face it, if that is the case, you'll have almost no time to practice. And like it or not you need practice! Even Richard Feynman did. Practice without paying attention to the clock until you feel comfortable with your material. Once you are comfortable, practice some more until that material fits in the presentation's time slot. Finally practice some more until you deliver your material in less (1 or 2 minutes) than the original time. When you are onstage your time perception is not the same as your audience's. You might feel you have only talked for 5 or 10 minutes, when in fact the audience has been listening for 20 or 25. One last thing, all this practice will hopefully lead to further trimming and editing of your talk and visuals. Don't worry, that is a good thing.