How do I adjust the frequency of forum emails?

Go into your user preferences while on the forums:

Select "Emails" on the left menu: 

Change things around as you see fit! We recommend asking for an activity summary on a daily or weekly basis to keep up with what's going on. You'll get less emails overall that way.

Note: emails are different from forum notifications. Notifications are just push notifications that happen on the forum itself, similar to any other notification you may get on a social media platform. Emails actually arrive in your inbox. =)