This cron job setup you can use for file retention policy, trashbin cleanup etc.
To check existing cronjob for user www-data:
crontab -u www-data -l
To edit for add new cronjob:
crontab -u www-data -e
Add this script at the bottom of line:
*/5 * * * * php -f /var/www/nextcloud/cron.php
Verify if the cron job has been added and scheduled:
crontab -u www-data -l