I always record in .flv for crash safety. After recording I cut and remux them to .mp4 using avidemux.
However when I create a big file avidemux seems to get the wrong duration, this only appears to happen with relatively large files.
The first time I experienced this was when I recorded lossless (qp=0) and got a 8GB file with a duration of 20 minutes, avidemux falsely though the file was only 9 minutes long. I figured it was due to recording lossless, so I switched to crf=18 instead. I then recorded another video with the new settings that came in at around 5GB with a duration of 1 hour 14 minutes. avidemux thinks its 18 minutes long and thus only reads the first 18 minutes.
Thanks for your help!
Please state the exact version of Avidemux and the platform it is running on. What does ffprobe report for your flv videos? Is ffplay capable of playing them correctly?