It greatly depends on the file size, regardless of the original film length. I have worked with film footage that was 30 seconds long and was over 2GB big, so expect funky things in the world of film editing. Check the file size and if it is too big, try compressing the video file before importing it into iMovie or change the file type completely (.mov to .mp4, whatever), depending on the sort of quality you want to achieve. I hope I helped.
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