You can change the duration of a transition. It can be as long as the shorter of the two clips. To make a transition longer, drag the beginning of the transition toward the beginning of the timeline. To make it shorter, drag the beginning of the transition toward the end of the timeline.
As you drag, a small text box will open by the mouse displaying the duration of the transition, so you can adjust to the exact length you want.
You can also change the default length of all transitions by going to Options>Advanced>Transition duration.
