How frequently does Terra background delivery fire?

Terra's Background delivery for iOS uses background app refresh to run queries in the background. Background delivery runs as long as: the user has enabled background app refresh in their settings and their phone is not on low-power mode.

Even with these conditions met, how frequently iOS triggers background delivery for your app would depend on many factors e.g: how many resources your app uses, how frequently your user interacts with your app, when the user uses their phone most often

All of these factors and many others are used to determine the interval of firing background delivery, and it is solely under iOS and Apple's control whether your app is favoured for frequent syncs.