As soon as the email notification pops up, I check it to know what it is all about. There are some official mails that shouldn't be left unattended to for long.
I get a lot of mail but around 90% of it is spam and that is entirely due to vBulletin's data breach in 2015. As a result I check my mail multiple times a day just to keep it under control.
Since that breach I've taken to using a unique email when I sign up to a new site so I know if a particular site has had a breach or sold my details on.
I don't know. Probably after a few hours or so.
It's at least 10 gives a day. I used to get notifications from my Gmail app... But for some reason they are not working at the moment
If I get an email notification on my phone, I always drag down the notification pane to see what kind of email that is. That is how I always check my email immediately.