You can use the select filter for a set of custom ranges:

ffmpeg -i in.mp4 -vf select='between(t,2,6)+between(t,15,24)' -vsync 0 out%d.png

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