An ethical question...

Given below is the conversation between me and my friend.

Scene: Office. It is 5:42 PM and I'm making an attempt to go home.

Me: Chalo… let me pack up. I am leaving for the day
Friend: Now! So early! It is just 5:42!
Me: Yes it IS 5:42! Time to leave! I'm leaving on time. It is not early.
Friend: Did you come at 9:42? Worked for 8 hours?
Me: Wrong question. You should be asking "Did you complete your work?"
Friend: But you are billing for 8 hours, aren't you?
Me: What matters is 8 hours worth of effort. If you can complete the job in four hours… then good!

The above conversation is a light one with each of us pulling other's leg. Both of us work in a company where working late in the night is the de-facto norm that if you work between 9 AM and 6 PM, people will start staring at you as if you have done something terrible.

The question I have is: Is it ethical to leave office early if you have completed your work for the day. You are completely aware that you are billing for more hours than what you have really done. But then, had it been someone else in your place, they would have taken more time to complete the task/work.