You can display the timestamp for each frame with ffprobe with awk to only output key frame info. Works in Linux and macOS.
ffprobe -loglevel error -select_streams v:0 -show_entries packet=pts_time,flags -of csv=print_section=0 input.mp4 | awk -F',' '/K/ {print $1}'
Or a slower method that works on any OS and does not require awk or similar additional processing tools:
ffprobe -loglevel error -skip_frame nokey -select_streams v:0 -show_entries frame=pkt_pts_time -of csv=print_section=0 input.mp4
Results:
0.000000
2.502000
3.795000
6.131000
10.344000
12.554000
16.266000
17.559000
...
See the ffprobe documentation for more info.