Gnome Eye Care is a Gnome extension that helps you to follow the 20-20-20 rule by remind you to take the breaks each 20 minutes.
$ apt install libnotify4 gir1.2-gsound-1.0
$ git clone https://gitlab.com/gnumoksha/gnome-eye-care ~/.local/share/gnome-shell/extensions/gnome-eye-care@gnumoksha.gitlab.io
$ gnome-extensions enable gnome-eye-care@gnumoksha.gitlab.io