There's an ideal amount of time you can spend at work, working. In fact there's more than one ideal amount of time. In my case, I find that if I work for 6 hours and then go home, I still have enough mental energy left to work on personal projects after the commute. Working 8 hours is also good, although productivity does decrease a lot in the later hours. But it's better than working 7 hours, because in that case I find myself both mentally tired and not with enough time and mental energy to do stuff at home.