I'm in my second semester in my biology PhD. I do research in the lab, take classes, teach, and have obligations like meetings with my advisor and lab meetings every week. Honestly, last semester I found it hard to get more than 40 hours a week. I go home around 5 and don't do anything school related, including checking emails, until I get back around 8 the next morning. Occasionally I've had to come in on the weekends for experiments, but those weeks, I leave a couple hours earlier on the Friday before. I had to grade on the weekend twice last semester, but only because I wanted to get my students feedback on the drafts before their final lab reports were due.
I don't expect every week for the rest of my PhD to be like that, but if other students in my department (like the guy who's the president of a minority students association, the treasurer for the graduate senate, and a 4th year student who also teaches) can manage to only work 40-45 hours a week, I don't think it's absurd for me to want to continue treating it like a 40 hour a week job.
I will say that a lot of things besides your own work habits or time management figure into being able to do that though. My department is the kind where I feel completely comfortable saying, "Sorry, I'm not staying past 5:30," because there isn't a culture of pressuring people to work more and my advisor doesn't expect me to work myself to death, so I'm really thankful for those things. It's also different if you're approaching a deadline. I think it's normal to work more to finish something that's coming up, but every week shouldn't be like that.