How do android check for apps notifications? Can I change the frequency between checks? I want to make my phone (6.0.1) check for notifications each hour (instead of each minute, or whatever it's set as default). Is it possible?

I'd like to change this value for notifications in general.
There is more than one kind of notification. 


Poll: IMAP email normally uses polling. Your mail application checks for mail periodically, and if there is a new message it displays a notification. The polling interval is configurable, but this is on a per-account basis. 
Push: Your Android device maintains a connection to Google's cloud messaging server (Firebase or GCM). All applications use this single connection. When an application server sends your device a message, it is relayed through the cloud messaging server and given to the corresponding application, which handles it immediately. As far as I know, there is no way to limit when this occurs short of disabling the data connection. 


In summary, you can change poll notification frequency as the applications allow, but not push notifications.