Maybe you shouldn't know how much time you're wasting on Facebook but, here's how you can find out anyway.
This is currently only available through the Mobile Apps on your device. iPhone is what I'll use to you show here but, is also the process for Android devices.
Open the Facebook App and select More (3 lines icon) > Settings & Privacy > Your time on Facebook.
You can then tap on any day within the last week to see how much time you have spent on any given day on that device. Unfortunately this feature doesn't as yet allow you to see the combined time spent across all devices you may have, so you would need to repeat this process for each individual device.
If you are indeed concerned about how much time you are wasting, then the app will allow you to Set Daily Reminder, which will then instruct the app to alert you when your allotted time has passed. It won't stop you from using the App, just a friendly reminder.