Been on both side (employee/employer). For creative jobs, its difficult to say when a task is finished. You can Finish it in 2 hours with "OK" result, or spend some more time on it to improve it.

You can continue to deliver mediocre results, cutting corners and say "See! I could do my 40 hours of work in 20 hours".

Then one day you get fired and say "See! these CEOs are so heartless they don't care".

I feel both need to set right expectations - it depends on the kind of job. If the expectation(contract) is to give x hours of your day to something, you should.

If expectation is to get the results and you can do it in quarter of the time that they expected, then great!