You change the timing of your slideshow in Playback. You can change the duration of slides (how long each one stays on-screen) from 0–20 seconds, as shown in Figure 1. Twenty seconds is a very long time when you consider this is how long each image stays on the screen. That might be something to use for a slide that is just a background to an event, however.
Transitions between photos also range from 0–20 seconds—I am not sure what you would do with a slideshow that had 20 seconds for each slide and 20 seconds of transition to change between them.

Figure 1: Change the timing of your slideshow in Playback.
How long or short the slides stay on-screen and the length of transitions is very subjective. You need to preview your show and see how the timing feels, based on what you are trying to accomplish with the slideshow. In addition, timing may be limited by your computer. Lightroom has to find and load each image from the hard drive as it plays a slideshow. If your hard drive is slow, your RAM amount too small, or your processor speed too slow, you may find the show does not play properly at short duration times.
Here are some things affecting your timing choices that you might want to consider:

Figure 2: One image in a slideshow of detailed landscapes.

Figure 3: Changing images quickly works best when showing a series of action photos.
Timing, and all of these ways of thinking about timing, affects your choice of how long a slide is on-screen as well as how long the transition is between two slides. Usually, you will combine short times for an image to appear on-screen with short transition times. Conversely, you will typically combine long times onscreen with longer transition times. There are no absolute rules about this, however. Try different settings with your images, click Preview, and see how they affect the show.