No option for that. Click the chevron at the top-right of the VP window and file a suggestion.
However, you can program a template with the transparency effect that has the desired duration. Create the curve, then click the white & blue Save folder on the effects toolbar and name it.
It will look something like this... 
When you want that fade click the fx button on the clip or VIDEO EFFECTS on the toolbar, scroll down to the templates area and click on your saved effect. It will be applied.
You can also drag the edge of a fade transition on the timeline, to change its duration.