Before you can begin measuring the number of people who use your app you have to define what “using your app” means. Meaningful usage implies that people are using your product to solve the problem they signed up to deal with.
Meaningful usage for Airbnb would be booking a night. For WhatsApp, it’s sending a message. If your app does rideshare then it’s probably booking a cab. If you deliver food then it’s likely ordering dinner.
Once you have your core action then you need to figure out your usage interval. Your usage interval is based on how often the problem you solve naturally occurs.
If Airbnb looked at how many people booked nights a week, it might look like nobody uses their app more than once. People go on holiday once, maybe twice a year. It makes more sense to look at how many people book nights a year. What looked like lots of inactive users, now looks like lots of repeat business.
Once you have your core action and your usage interval you can construct a cohort chart.
The first column in a cohort chart corresponds to your usage interval. If your usage interval is monthly then it groups people by the month they signed up. The rows show you what percentage of people continue to perform the core action each month after they signed up.
Then average out all the columns out and plot them on a curve.
Now you know how to work out the exact percentage of people than continue to use your product 6 months weeks after they sign up.