For me, I take a break when my sleep pattern starts to suffer. This happens around once a week. I can' t sleep because I am thinking too much. I try to pre-plan when I will take a break 2-3 days before. So in the back of my mind, I am mentally prepared for the break.
I am still finding way to relax during my break though.
I am asking because I wonder if other founders have a more efficient way to determining when to take a break that I could learn.