I've found that I get exponential returns on the time I can _effectively_ dedicate to work. For example, I'll get more than twice as much done in 20 hours/week than I will in 10. For me, the sweet spot is higher than 40 hours. I'll get a lot more done in 50 hours than I will in 40. I have the extra time to step back, learn, work on tools, improve the process, etc. At just 40, I'm usually chasing my weekly goals, only getting through the most important tasks. So, as far as my career goes, I'm mostly treading water at 40 hours. But when I spend a few months at 50, I end up having a ton of really cool results that I need to differentiate myself in a review. And also, work is simply a lot more fun when I have more than enough time to get my core work done. I end up more involved in the broader project and enjoy things a lot more. If I tried to just get by with 35 hours/week, I'd end up being stressed out constantly, hating my work, and ultimately getting poor reviews.
So yea, I think everyone needs to find the right number of hours to hit their own personal sweet spot. Then, take on a conservative amount of work that leaves around 20% of their time available for good ideas and helping out more broadly. Don't start the week by planning 50 hours and over-promising. Leave yourself extra time to improve yourself. Over months and years, that extra time adds up and results in a highly successful career.