How to extract duration time from ffmpeg output?

ffmpeg is writing that information to stderr, not stdout. Try this:

ffmpeg -i file.mp4 2>&1 | grep Duration | sed 's/Duration: \(.*\), start/\1/g'

Notice the redirection of stderr to stdout: 2>&1

EDIT:

Your sed statement isn't working either. Try this:

ffmpeg -i file.mp4 2>&1 | grep Duration | awk '{print $2}' | tr -d ,

You can use ffprobe:

ffprobe -i <file> -show_entries format=duration -v quiet -of csv="p=0"

It will output the duration in seconds, such as:

154.12

Adding the -sexagesimal option will output duration as hours:minutes:seconds.microseconds:

00:02:34.12