Edit: Later, I found a much easier solution using Cross Dissolve (see
https://apple.stackexchange.com/a/268993/50985 ). So I don't recommend these steps anymore, but I'm leaving it here for educational purposes.
No, you cannot change the fade time in
Settings > Fade Out to Black.
Even the manual
Transitions > Fade to Black does not work well, it still fades too quickly. It already reaches black merely 33% into the duration of the transition. I spent hours and figured out a workaround (messy but achieves the desired result).
Let's say you want a fade to black in the last 5 seconds of your video. To achieve that, you will need to set the
Transitions > Fade to Black to 15 seconds so that it will fade to black in 5 seconds. Then we will throw away the extra 10 seconds (not trivial).
- Add your clip to the timeline.
Modify > Detach Audio. (This is necessary because the audio fades slower than the video. The video is black around 33% into the duration, whereas the audio can still be heard about 90% into the duration).
- Go to the end of the audio clip and drag the audio handle to the left to create a 5s audio fade out.
Modify > Add Freeze Frame. Then click on the freeze frame and change the Duration to 10s in the preview panel. (This adds the extra 10 second pad at the end).
- Drag the
Transitions > Fade to Black effect to the end of the timeline. Double click on it to change the Duration to 15s. (The timeline will look messed up, but it's okay).
- Play your video and make sure the fade to black is satisfactory (video and audio). There should be an extra 10s of black silence at the end.
File > Share > File to export an mp4 video.
- Open the mp4 video with QuickTime.
Edit > Trim. Drag the right handle left by 10s and click Trim.
File > Save to save the trimmed mp4 file. (I'm pretty sure this is a lossless trim because it saved a 100MB video instantly).
- Now view the final mp4 video to make sure everything is right.
See how easy and intuitive Mac software is? (I did the same thing in Windows Movie Maker in one step).