Calculate it based on time, instead.

That's what I do and it works great for me, and many others. First, find the length of the video in the below snippet:

Seems stream 0 codec frame rate differs from container frame rate: 5994.00 
(5994/1) -> 29.97 (30000/1001)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/Users/stu/Movies/District9.mov':
  Duration: 00:02:32.20, start: 0.000000, bitrate: 9808 kb/s
    Stream #0.0(eng): Video: h264, yuv420p, 1920x1056, 29.97tbr, 2997tbn, 5994tbc
    Stream #0.1(eng): Audio: aac, 44100 Hz, 2 channels, s16
    Stream #0.2(eng): Data: tmcd / 0x64636D74

You'll should be able to consistently and safely find Duration: hh:mm:ss.nn to determine the source video clip size. Then, for each update line (CR, no LF) you can parse the text for the current time mark it is at:

frame=   84 fps= 18 q=10.0 size=       5kB time=1.68 bitrate=  26.1kbits/s    
frame=   90 fps= 17 q=10.0 size=       6kB time=1.92 bitrate=  23.8kbits/s    
frame=   94 fps= 16 q=10.0 size=     232kB time=2.08 bitrate= 913.0kbits/s    

Just be careful to not always expect perfect output from these status lines. They can include error messages like here:

frame=   24 fps= 24 q=-1.0 size=       0kB time=1.42 bitrate=   0.3kbits/s    
frame=   41 fps= 26 q=-1.0 size=       0kB time=2.41 bitrate=   0.2kbits/s    
[h264 @ 0x1013000]Cannot parallelize deblocking type 1, decoding such frames in
sequential order
frame=   49 fps= 24 q=26.0 size=       4kB time=0.28 bitrate= 118.1kbits/s    
frame=   56 fps= 22 q=23.0 size=       4kB time=0.56 bitrate=  62.9kbits/s    

Once you have the time, it is simple math: time / duration * 100 = % done.

Since my comment got a few upvotes, I figured I'd leave it as an answer:

ffmpeg -i 00000.avi -map 0:v:0 -c copy -f null -y /dev/null 2>&1 | grep -Eo 'frame= *[0-9]+ *' | grep -Eo '[0-9]+' | tail -1

This should be fast, since no encoding is being performed. ffmpeg will just demux the file and read (decode) the first video stream as quickly as possible. The first grep command will grab the text that shows the frame. The second grep command will grab just the number from that. The tail command will just show the final line (final frame count).

Not all formats store their frame count or total duration - and even if they do, the file might be incomplete - so ffmpeg doesn't detect either of them accurately by default.

Instead, try seeking to the end of the file and read the time, then count the current time while you go.

Alternatively, you can try AVFormatContext->nb_index_entries or the detected duration, which should work on fine at least undamaged AVI/MOV, or the library FFMS2, which is probably too slow to bother with for a progress bar.

Try something like:

ffmpeg -i "path to file" -f null /dev/null

It writes the frame number to stderr, so you can retrieve the last frame from this.

Sorry for the necro answer, but maybe will need this (as I didn't found a solution for recent ffmpeg releases.

With ffmpeg 3.3.4 I found one can find with the following:

ffprobe -i video.mp4 -show_streams -hide_banner | grep "nb_frames"

At the end it will output frame count. It worked for me on videos with audio. It gives twice a "nb_frames" line, though, but the first line was the actual frame count on the videos I tested.

Cmd ->

ffprobe.exe -v error -select_streams v:0 -show_entries stream=r_frame_rate,duration -of default=nw=1 "d:\movies\The.Matrix.1999.1080p.BrRip.x264.YIFY.dut.mp4"

Result ->

r_frame_rate=24000/1001
duration=8177.794625

Calculation ->

Frames=24000/1001*8177.794625=196071

Proof -> 

ffmpeg -i "d:\movies\The.Matrix.1999.1080p.BrRip.x264.YIFY.dut.mp4" -f null /dev/null
ffmpeg version N-92938-g0aaaca25e0-ffmpeg-windows-pacman Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8.2.0 (GCC)
  configuration: --pkg-config=pkg-config --pkg-config-flags=--static --extra-version=ffmpeg-windows-pacman --enable-version3 --disable-debug --disable-w32threads --arch=x86_64 --target-os=mingw32 --cross-prefix=/opt/sandbox/cross_compilers/mingw-w64-x86_64/bin/x86_64-w64-mingw32- --enable-libcaca --enable-gray --enable-libtesseract --enable-fontconfig --enable-gmp --enable-gnutls --enable-libass --enable-libbluray --enable-libbs2b --enable-libflite --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopus --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libzimg --enable-libzvbi --enable-libmysofa --enable-libaom --enable-libopenjpeg --enable-libopenh264 --enable-liblensfun --enable-nvenc --enable-nvdec --extra-libs=-lm --extra-libs=-lpthread --extra-cflags=-DLIBTWOLAME_STATIC --extra-cflags=-DMODPLUG_STATIC --extra-cflags=-DCACA_STATIC --enable-amf --enable-libmfx --enable-gpl --enable-avisynth --enable-frei0r --enable-filter=frei0r --enable-librubberband --enable-libvidstab --enable-libx264 --enable-libx265 --enable-libxvid --enable-libxavs --enable-avresample --extra-cflags='-march=core2' --extra-cflags=-O2 --enable-static --disable-shared --prefix=/opt/sandbox/cross_compilers/mingw-w64-x86_64/x86_64-w64-mingw32 --enable-nonfree --enable-decklink --enable-libfdk-aac
  libavutil      56. 25.100 / 56. 25.100
  libavcodec     58. 43.100 / 58. 43.100
  libavformat    58. 25.100 / 58. 25.100
  libavdevice    58.  6.101 / 58.  6.101
  libavfilter     7. 47.100 /  7. 47.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'd:\movies\The.Matrix.1999.1080p.BrRip.x264.YIFY.dut.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.25.100
  Duration: 02:16:17.91, start: 0.000000, bitrate: 2497 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x800 [SAR 1:1 DAR 12:5], 2397 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 93 kb/s (default)
    Metadata:
      handler_name    : GPAC ISO Audio Handler
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> wrapped_avframe (native))
  Stream #0:1 -> #0:1 (aac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, null, to '/dev/null':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.25.100
    Stream #0:0(und): Video: wrapped_avframe, yuv420p, 1920x800 [SAR 1:1 DAR 12:5], q=2-31, 200 kb/s, 23.98 fps, 23.98 tbn, 23.98 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      encoder         : Lavc58.43.100 wrapped_avframe
    Stream #0:1(und): Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s (default)
    Metadata:
      handler_name    : GPAC ISO Audio Handler
      encoder         : Lavc58.43.100 pcm_s16le
frame=196071 fps=331 q=-0.0 Lsize=N/A time=02:16:17.90 bitrate=N/A speed=13.8x
video:102631kB audio:1408772kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

linux

ffmpeg -i "/home/iorigins/Завантаження/123.mov" -f null /dev/null

ruby

result = `ffmpeg -i #{path} -f null - 2>&1`
r = result.match("frame=([0-9]+)")
p r[1]

The problem with ffprobe and ffmpeg info is that the actual length in frames differs by some number.

This script tries to extract the last frames. The frame number that succeeds works inside blender, too. Frames beyond that number cannot be extracted in blender, neither.

#!/usr/bin/env bash
# find the number of frames in a movie clip
FMAYBE=$(ffprobe -v error -select_streams v:0 -show_entries stream=nb_frames -of default=nokey=1:noprint_wrappers=1 $1)
let FMAYBE=$FMAYBE+1
FEMPTY="-"
while [ -n "$FEMPTY" ] ; do
  let FMAYBE=$FMAYBE-1
  echo "Trying $FMAYBE"
  FEMPTY=$(ffmpeg -i $1 -vf select="between(n\,$FMAYBE\,$FMAYBE)" -vsync 0 /tmp/fmaybe%d.png 2>&1 | grep 'empty')
done
echo "Succeeds: $FMAYBE"

Get duration in seconds and multiply by frame rate. (This method is fast)

Get duration:

ffprobe -i input.mkv -show_entries format=duration -v quiet -of csv="p=0"

Get frame rate:

ffprobe -v 0 -of csv="p=0" -select_streams V:0 -show_entries stream=r_frame_rate input.mkv

If the answer has a fractional part then just truncate it.