There is no refresh rate setting in Pure calendar or Pure Grid calendar because it refresh only when calendar database is modified.
This database is modified only when you add/remove/modify an event.
So, generally, 1 to 10 refresh per day.
Each refresh use the battery during ~1 second.
Now, there is 1 cause possible of higher refresh rate : a calendar database corruption.
It leads to periodic refresh made by Android calendar sync service (not from the widget).
THAT drain the battery … with or without widgets.
If there is no such corruption, the battery draining is bellow 1% per day.