Facebook has been refined over the years to keep you online and interacting with the service as long as possible. That ensures you'll see lots of Facebook ads, but it can be a drain on your time if you get stuck in a Facebook vortex. With everyone suddenly concerned about limiting screen time, Facebook has rolled out its previously announced "Your Time On Facebook" feature. It tells you how long you've been using the Facebook app, but that's pretty much it.
You'll find this feature tucked away under Facebook’s More tab > Settings & Privacy > Your Time On Facebook. There is, unfortunately, not much going on here. There's a graph (which you can see below) that provides the total time you've used Facebook for the last seven days. You can tap on any day to get the exact time, but this is a bad graph—there's no y-axis, so you can't tell much without tapping on all the bars.
So far, this has been live in every Facebook app install we've checked. That's the only place you'll see it, though. The website doesn't currently have a similar feature. It wouldn't do you much good anyway as Facebook hasn't included any features to warn you about usage or block access when you've hit your limit. You're further ahead to use Digital Wellbeing if you're lucky enough to be on Android Pie.