Before you insert your transition, go to the top and click on 'tools' and then
on 'options'. Choose the 'advanced' tab and you will see where you can
adjust
the picture duration and also the TRANSITION duration. Choose a longer time
to make it last longer.
The reply from dickmr is correct for what you want to do. have you tried it
? The transition duration will determine how slow or fast the fade happens.
if the clips are already on the timeline then you can adjust the transition
duration by changing to the Timeline View (press Ctrl-T) and then dragging
the right-clip over the left-clip thus increasing the overlap. The fade will
last as long as the overlap.