Older video files can be removed using tmp-reaper
sudo apt-get install tmpreaper
You will need to acknowledge the security warning.
Install a cron job under the motion
user to remove video files older than 180 days.
sudo crontab -e -u motion
In the crontab, add a daily job.
# remove video files older than 180 days
0 0 * * * /usr/sbin/tmpreaper 180d /home/motion >/dev/null 2>&1