Drop this into a crontab with your desired frequency:

  DISPLAY=:0 import -window root "$HOME/Pictures/screenshot-$(date '+%Y%m%d%H%M').png"

Requirements

ImageMagick installed

Improvements

You can change the date +FORMAT for a more readable filename. See the date(1) manpage.

Caveats

You may need to change your DISPLAY to match your actual X11 server. You may need to add XAUTHORITY=$HOME/.Xauthority depending on your X11 ACLs. Probably doesn't work with Wayland, but nothing works with Wayland so YMMV.