There is a Screen Time introduced by iOS 12 that will help you in keeping track of how you are actually spending the time on the device. The main question is that can you put the widget in the TODAY VIEW to access it quicker?
Yes, there is a widget available for Screen Time. You will swipe right on the iPhone screen so you can get to Today View. Then scroll down to bottom and tap the EDIT option. Scroll down to the option of Screen Time and tap the + button that is in green color.