The duration of each sample to be added. You must specify this parameter in the media's time scale. For example, if you are adding sound that was sampled at 22 kHz to a media that contains a sound track with the same time scale, you would set durationPerSample to 1. Similarly, if you are adding video that was recorded at 10 frames per second to a video media that has a time scale of 600, you would set this parameter to 60 to add a single sample.
So simple, right? 60/600 = 1/10. So if I have different framerates, I have y/600 = 1/x.
f(x) = 600/x.
So for a movie at 24fps, X=25.
Likewise, 25fps, x=24.
And 30 fps, x=20.
No really, it makes sense. Really.