Quality is better than quantity. If you can revise well 3 hours per day it’ll be better than doing 6 hours where you can barely concentrate. The best thing is to work out a plan for what and how you want to revise so you can use the time you do revise well. That way, you can look after yourself at the same time. Even as a 4th year Cambridge student there are days where I’m just not feeling the work and don’t want to and can’t do it well, and other much more productive days. The best thing I’ve found is to go with the flow a bit and do what you can do on a particular day, and just use that time well.