You can make a title appear over as much or as little video as you want. You set duration individually for each title in your project.
To adjust title duration:
In the Project browser, double-click the blue or gold title bar.
In the inspector that opens, type how long you want the title to stay onscreen, in seconds, in the Duration field.
If iMovie is set to display timecode values for project clips, type the duration using the following convention: HH:MM:SS:Frames (hours, minutes, seconds, frames).
To adjust how quickly the titles in your project fade in and fade out, select Manual and move the slider to the left to make titles fade in and out more quickly or to the right to make them fade in and out more slowly. This setting applies to all titles in your project. Animated titles can’t be set to fade in and out.
You can also move your pointer over either end of the blue or gold title bar in the Project browser. When it turns into a resize pointer, drag to the right or left to lengthen or shorten the duration.