This depends on your goals. If you have personal goals and desires outside of your job, the 40 hr week can be really restrictive and prevent you from really pursuing the things you care about. However, if you just want to hold down a job and cover the bills, 35-40 hrs/week is quite easy to do and still have plenty of leisure and self-care time.
I, myself, and full of ideas and curiosity, and have trouble finding the time to devote to all of the things I want to explore and concepts I want to build out. I work around my 40 hour job and still manage to stay in shape, spend quality time with my wife, pitch-in with the housework, and chip away at a few personal projects, but this does make for a fairly busy schedule and I must admit that I've had to adopt some daily practices for fostering mental and emotional consistency and control in order to keep this lifestyle.
As of right now, my aim is on holding down my current job to invest in things that can reduce my expenses and hopefully free myself up to work part-time and build up more independent (and more passive) styles of making money (b/vlogging, airbnb, writing ebooks, flipping websites, etc) so that I'm freer to work on some of my own ideas in software that I really believe in.