5.30 is the earliest I've been in... and that was a Saturday too!
But generally I'm a lateish starter, I like to do things in the morning (like exercise, read, clean...) so I usually get in around 9 - 10.30 and leave around 6-7 on a normal day, or 8.30-10.30 pm when I was writing up and writing papers.
The latest I have stayed is 00.30 I think.
I kind of like it when there's no-one around too actually!
To get out a slump, I just keep working, I make it to coffee time, I make it to lunch, I make it to dinner, and slowly but surely the day is over and I can go home. Eventually it becomes easier and the slump is over.