# Recursive all files in dir
exiftool -ext mp4 -r -n -q -p '${Duration;$_ = ConvertDuration(our $total += $_)}' . | tail -n 1

# More control
# calc-durs Replay_* - all files matching glob in single folder only
# calc-durations -r Replay_*   - recursive in all folders
calc-durs() {
    local -a files
    if [[ "$1" == "-r" ]]; then
        shift
        exiftool -ext mp4 -r -n -q -p '${Duration;$_ = ConvertDuration(our $total += $_)}' -- "${@:-.}" | tail -n 1
    else
        files=("$@")
        exiftool -n -q -p '${Duration;$_ = ConvertDuration(our $total += $_)}' -- "${files[@]}" | tail -n 1
    fi
}