https://techcrunch.com/2018/11/20/see-time-spent-on-facebook/

15 weeks after Facebook announced its “Your Time On Facebook” tool that counts how many minutes you spend on the app, the feature is finally rolling out around the world. Designed to help you manage your social networking, the dashboard reveals how many minutes you’ve spent on Facebook’s app on that device each day for the past week and on average.

You can set a daily limit and receive a reminder to stop after that many minutes each day, plus access shortcuts to notification ,News Feed, and Friend Request settings. Those last two shortcuts are new, but otherwise the feature works the same as when it was previewed. You can access it by going to Facebook’s More tab -> Settings & Privacy -> Your Time On Facebook.