Work-life balance is really what you make of it in DFM R&D, as well as how well you're able to get your work done. The expected hours to be at work for my team was 10AM-4PM, but often times you need to be in earlier to get work done or attend meetings, and more likely than not, you'd need to stay later than 4PM to troubleshoot and solve the problems at hand. I personally thought that the work-life balance was the worst part at Apple. Officially, no one cares when you get to or leave work, but people who work at Apple are very passionate and love the products, so leaving earlier than everyone else is sometimes frown upon. With that said though, there were interns who got into work at 10 and left at 5, but there were others who stayed all the way till 9PM pretty regularly. It's really what you make of it, and how well you're able to communicate your project status to your team so they don't get the wrong idea if you do tend to prefer normal working hours.
You're given a lot of work as an intern in PD, so it really depends on how well you communicate with your manager. You meet with your manager on a regular basis, usually to go over goals for your internship and check up on your workload, though they can also offer you plenty of career advice. You're by no means expected to work 12 hour days, but plenty of interns worked 10+ hours a day because they were incredibly energetic, creative, and took pride in their work. Just a heads up, 40 hour work weeks are rare after your training weeks are over, so make the most of them!