It’s possible, but unfortunately it hasn’t made it into the user interface. There is a column for it in the database, but you have to set it manually in the SQL console.
But to make things easy, I wrote a bash script, that can set the refreshrate of your described calendars to your choicy.
Install the script:
sudo wget -qO /usr/local/bin/nc-subscribed-calendar https://global-social.net/script/nc-subscribed-calendar
sudo chmod +x /usr/local/bin/nc-subscribed-calendar
Now simply run it:
nc-subscribed-calendar
The script wil guide you through the steps.
Restriction: Only for mysql
[Edit]: for MySQL and PostgreSQL
Much and good luck,
ernolf