To determine the length of the music clip, you can follow the steps below:

1. Open the Douyin application and click the " " button on the homepage.

2. Select the "Select Music" option.

3. Select the background music you want to use in the music list.

4. After selecting the music, click the scissors icon behind the music name.

5. Use the audio axis at the bottom of the screen to drag to select the start and end positions of the music.

6. After adjusting to a satisfactory position, click the check button to save the settings.