As I pointed out, Automatically only works when the iPad is plugged into power, and has a wi-fi connection. I do not know how often it checks, but I would guess it’s more frequent than 15 minutes. Frequently enough that Apple believes it would impact your battery and cellular usage; thus the limitations.
I use Automatic, but since only my secondary accounts are set to Fetch, I don’t care if I get timely notifications for them. My iCloud account is Push, so notifications are quick. Even then, I have my email notifications turned off for all but my VIP contacts.
I find the constant barage of notifications irritating, especially spam and/or other unimportant ads and newletters.