Hello,
I found in this guide that as an administrator it was possible to modify the synchronization interval of the subscriptions on the calendar, but I can’t find this setting…
https://docs.nextcloud.com/server/21/user_manual/fr/pim/calendar.html
AFAIK this cannot be changed in the WebUI, but you can change it from the command line using occ: Calendar — Nextcloud latest Administration Manual latest documentation