Mini in-app messages display time and customization ability are slightly different depending on which platform you’re targeting:
On iOS, the default display time for mini in-app messages is six seconds. It's entirely possible to extend (or shorten) that display time via miniNotificationPresentationTime; for example, here the default has been changed to 12 seconds:
Mixpanel *mixpanel = [Mixpanel sharedInstanceWithToken: YOUR_MIXPANEL_TOKEN]; mixpanel.miniNotificationPresentationTime = 12.0;
On Android, the default display time for mini in-app messages is 10 seconds, and this unfortunately cannot be customized without altering the Mixpanel Android SDK at this line. Note, however, that the Mixpanel support team generally recommends against forking the SDK whenever possible.