iOS

Ask for a rating only after the user has demonstrated engagement with your app. For example, prompt the user upon the completion of a game level or productivity task. Never ask for a rating on first launch or during onboarding. Allow ample time to form an opinion.
Don’t interrupt the user, especially when they’re performing a time-sensitive or stressful task. Look for logical pauses or stopping points, where a rating request makes the most sense.
Don’t be a pest. Repeated rating prompts can be irritating, and may even negatively influence the user’s opinion of your app. Allow at least a week or two between rating requests and only prompt again after the user has demonstrated additional engagement with your app.
Prefer the system-provided prompt. The system's rating prompt offers a familiar, efficient experience that's designed to engage the user with minimal impact.
Don't use buttons or other controls to request feedback. Since the system limits how often rating prompts occur, attempting to request feedback in response to a control may result in no rating prompt being displayed.
The system automatically limits the display of the prompt to three occurrences per app within a 365-day period.
When you call this method while your app is still in development mode, a rating/review request view is always displayed so that you can test the user interface and experience. However, this method has no effect when you call it in an app that you distribute using TestFlight.

Android

Trigger the in-app review flow after a user has experienced enough of your app or game to provide useful feedback.
Do not prompt the user excessively for a review. This approach helps minimize user frustration and limit API usage (see the section on quotas).
Your app should not ask the user any questions before or while presenting the rating button or card, including questions about their opinion (such as “Do you like the app?”) or predictive questions (such as “Would you rate this app 5 stars”).