Anonymous
If it's a subscription based product, After usage of the product I check how many users continue with the subscriotion after 7 days, 15days, 1 month, 3 months and 6 month. The % of users who continue to have the subscriotion after the above time periods are the retained users
For non subscription products, I can check weekly or monthly retention by finding % of users who come back to product the week or month after using
