It really takes little effort.
The amount of space to be regained is purely subjective.
If one tends to install/uninstall packages frequently; perhaps including packages via the AUR; and if one fails to cleanup along the way; the space to be regained can potentially be quite substantial.
Example;
When a package is installed it may often install dependencies (only needed for that package). Many might uninstall a package while being completely oblivious to the need to also remove those dependencies.
Using:
sudo pacman -R <package-name>
will uninstall the package.
If it is a meta-package, it should also uninstall dependencies; otherwise, the following command tends to cleanup-as-you-go;
sudo pacman -Rns <package-name>