The manual does not tell how to, neither did the trial and error method.

All transitions are displayed with small yellow rectangles (or red for the currently selected one). To change the duration of a transition, you just have to drag one side of the transition, so that the rectangle fits the time you want for the transition.

There is currently no way to set the duration by entering a time, but I will implement this soon in svn.

Regards

Cool, thank you for the quick anwer!