Such a good question, yet there is no clear answer because each person is so unique. Personally I’ve worked 4-5 days per week, but after a few years it started wearing on me so I changed to 3 days per week. This may not be the case for you however! I work with a hygienist who has worked 5 days per week for over 30 years and she’s still going strong. The only way I can honestly answer this is by saying, listen to your body. A good operator chair, loupes, sharp instruments, and of course ergonomics will ultimately be your best bet for longevity.
I agree with Kara. I thought I would be disabled if I kept practicing as of last year. I put my DFV loupes away. Ordered SurgiTel loupes with a light, started Pilates and walking/jogging and have never felt better. The link below saved my career. I’ve even added a half day on Friday to my usual 4 day week schedule to help see new patients. The book practicing dentistry pain free will help you. Go to the link.