Currently the fade out time for all clips is set by default to 3 seconds.

Any attempt to change this results in huge times being inserted instead, or in the fade out producing silence.

Is there currently any way to change the fade out time in clips?

Which Kdenlive version are you using? That problem should be solved in 0.7.6 if I remember correctly

Drag the green area on the time line to the desired fade out time.

I'm using 0.7.3. I eventually found the green audio sliders on the top left and top right of each clip by accident.