When you tell people to take a break, tell them exactly when you plan to start again. Fifteen minutes is a typical length for a break. The shorter presentations – those under an hour, will generally not require a break, but if the presentation edges towards an hour and a half it may well be that offering a break in the middle can be the wisest thing to do.
Although people will sit for upwards of two hours in a movie theater, there is a clear difference between a movie and a presentation. Atmospherically, dynamically and in many other ways it is much easier to sit through a film of a certain length than it is to sit through a presentation of the same length.
If you do give a break to the audience at a presentation, it is essential to specify that you will begin the presentation again at a set time and impress upon them the importance of their being back at the right time.
If people stay out beyond the allotted time for a break then it simply results in the recommencing of the presentation being delayed, and gives a very bad impression. If there are some stragglers who take a little bit more time to arrive back, then it is beneficial to simply have a short, informal conversation with the people who have arrived on time or stayed in the auditorium during the break.
This can be a good way of gauging how the presentation is being received, and allow you to get an impression for what your audience is like.