Badly documented -

[mahen23]

Nowhere in the documentation does it explain what interval is (milliseconds or minutes) and does not define the default value if interval is not set. Documentation also does not specify what other parameters can use used to set into capacitor.config.ts for Background.runner.

[nblixt]

interval: The number of minutes after the the app is put into the background in which the background task should begin..

Looks like they updated the readme since you posted this.