Splitting files automatically
FFmpeg's segment muxer is designed for this.

ffmpeg -i in.mp4 -c copy -f segment -segment_time 1200 -reset_timestamps 1 in%d.mp4


Output files will be in0.mp4, in1.mp4...

Since the video keyframes in the source file may not be present exactly at the start of each 20 minute interval, the output segments won't be exactly 20 minutes each. Expect a deviation of a few seconds.