Official ffmpeg documentation on this: Create a thumbnail image every X seconds of the video
Output one image every second:
ffmpeg -i input.mp4 -vf fps=1 out%d.png
Output one image every minute:
ffmpeg -i test.mp4 -vf fps=1/60 thumb%04d.png
Output one image every 10 minutes:
ffmpeg -i test.mp4 -vf fps=1/600 thumb%04d.png