Ubuntu comes with a Workrave package. You can install it from the command line with the following command:
sudo apt-get install workrave
If you need a newer version than what is included in your distribution, use the official Workrave PPA.
The Workrave PPA can be enabled by entering these commands into a terminal:
sudo add-apt-repository --yes ppa:rob-caelers/workrave sudo apt update sudo apt install workrave
The Workrave PPA has separate package for each supported desktop environment. You only need to install the one for your desktop environment.
sudo apt install workrave-cinnamon sudo apt install workrave-gnome sudo apt install workrave-mate sudo apt install workrave-unity sudo apt install workrave-xfce