Is there a length limit?

MP3, WAV, OGG, Opus and FLAC files have no practical limit; they are processed in ten-minute pieces, so an eighteen-hour file is fine. M4A, M4B, MP4 and MOV are decoded in a stream on your computer, so a sixteen-hour M4B audiobook works too (the app shows "Decoding" with the time reached before the recording appears in the list). AAC, WebM, AIFF and CAF are decoded in memory and are limited to about 90 minutes each; for longer files in those formats convert them to MP3 or WAV first.