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.
Those are all for the refresh rate that affects ALL subscribed calendars.
An example: subscribed calendars with religious or legal holidays or moon phases can be upated once a month or even once a year while subscribed personal google callendars would have to be synchronized up to every 15 minutes or sometimes even more often depending on the individual case.
That is not possible with the setting described in the manual.
therefore I wrote the script to give the means to set it on a per subscribed calendar basis
Setting the default refresh rate, as explained in the manual, can only be done via occ, i.e. by administrators with console access.
My script gives an additional possibility to set a refreshrate separately for each subscribed calendar but as well only admin users with access to the database can make this changes. So my script doesn’t change anything on that, since it only can be run by an admin with terminal access.