Apple - How to set a refresh rate in a Subscribed Calendar

In "Settings->Mail, Contacts, Calendars->Fetch New Data" you can setup what to do. Depending on the account you can Fetch or Push updates to your iPhone - or choose to do it manually.

As of 2019/iOS 12.3.1, these settings are under Passwords and Accounts → Fetch New Data → Fetch. It looks like fetch settings are applied globally to all fetch accounts - you can't set different fetch options for individual calendars or mail accounts.