“Don't stay too late!”
You and your coworker are both really busy at work today. You worked later than usual, but now you're going home. Your coworker is still working. After you tell her that you're leaving, you say this.
Don't stay too late!
In spoken English conversation, you say "stay late" to talk about staying at work until later than usual, or later than other people:
Did you have to stay late?
The phrase "work late" is similar, but just a bit more formal.
This is a friendly phrase to use when you're leaving work and your coworkers are still working.