Settings->Battery -> Battery Usage, it brings up a bar chart in 2 hour intervals. You can click on each interval and it will give you a breakdown of % of battery expended by each app (along with how long that app ran), and if you scroll down to the bottom and click on 'System Usage' it'll give you more information, including screen time.
The only way to really know total screen time since last charge is to add up the individual segments (which will only be accurate if you were at a full charge with no screen time at the beginning of a 2 hour window).
DWB shos aggregate screen time on a daily basis. If you are done with it for the night and it's on the charger so that your first use of it for the day is when you take it off in the morning, that would be an easier way. I tend to stay up too late for that to work for me.