Using automatic refresh rate
An ad unit’s automatic refresh rate determines how often a new ad request is generated for that ad unit.
We recommend using the Google-optimized automatic refresh rate. The optimized rate is calculated using AdMob historical data to ensure the ads shown in your ad units are being refreshed at the best rate for banner ads.
You may also set a custom refresh rate of 30-120 seconds or disable refresh rate completely.
Refresh rate is disabled by default for native ads. Native ads should complement the content and environment surrounding them. Refreshing native ads regularly may interfere with the look, feel, and cohesiveness of an app.