Hi,
So it’s actually the dyndns plug-in and not duckdns.
I added the tag on the post to clearly identify the plug-in related to the question, please do not forget it in the futur.
Here the same question (in French)
About plug-in crons, generally speaking it cannot be adapted by end-user (you) unless explicitly foreseen in the plug-in.
Concretely for this plug-in you cannot change the frequency, as you find yourself, the best solution for you is to deactivate the cron and create a scenario to refresh on demand or with custom schedule.