How much of a song can I use in a project?

you can use the entire song during the project but make sure you cite the source you got it from and the original singer, song writer and the musician. However, in my opinion, during the project, make sure to use the song parts only where they are suitable. other wise the audience will lose their focus from the project and think about the song.

Moreover, its much better if you can use a soundtrack without a song as it is less of a distraction.