How much is enough? Are you productive during your 8 hours? Some organizations expect you to put in more than 8 hours. Not really for anything else but to show your dedication to working 12 plus hours each day.
While there are times when one might put in such long days (i.e. after hours maintenance or systems upgrades), I subscribe to the philosophy that if you cannot complete your task or make progress to complete your task then you are doing something wrong. If you are just putting in hours to show your face and not really accomplishing much then you are not efficient and fully maximizing your day.
Why not be efficient and accomplish your tasks in 8 hours or under? It leads to prioritizing work efficiently and the end result is a better quality of life.