Retention is highly dependent on the category of your app and your business vertical. Some apps like Facebook Messenger could see very high retention rates compared to an Amazon e-commerce app. Broad level retention are calculated by tracking your DAU/MAU, i.e., How many people return to your app over a given period. You can dig deeper by tracking the trend of DCU/MCU (Converting Users, in this case, the conversion action is something you can define for your business, e.g., for youtube it would be Video Views).
As far as avg retention rates goes, they’re pretty low industry wide. Only 2% of all users who install return to your app on the 30th day.
Original Quora question answered by Jay Dalvi, Product Evangelist for CleverTap