Hi
If you have a single clip, or are fading out the last clip, then the fade time you set will be what you get. A 2 second fade will be 2 seconds.
If you are applying the fade effect between two clips then the duration you set will be split between the end of the first clip and the start of the second clip. This means that if you set it for 2 seconds there will be a 1 second fade out and a 1 second fade in.
If you want a longer fade simply set a bigger duration. Fades between clips work correctly in version 3.79
If you want to fade in the start of a clip you should add a blank black frame before the clip and apply the fade transition to this.
You can use the transparency effect as Borate suggests, placing your clip (or clips) onto the overlay track (Videotrack 2) with a blank black frame beneath on Videotrack 1 at the fade points. Use the keyframe setting to alter the transparency level and duration of the transparency effect so the correct length of the clip appears and disappears against the black frame below. This is more complicated than using the straight forward Fade transition.
Nat