About 2.5 months ago, I began a workout routine and diet. I am/was overweight. I play poker full time. The working out, and diet has seemed to help my mental game. I have always had mental game issues, and I work on them as much as I can.
For me, I can grind (live poker) about 6 hours on A-game. After that, I really have to watch myself.
The challenge for me is staying in the zone. I am playing live 1/2 and 2/5. The players are all bad for the most part. Strategy is helpful, but the mental aspect I find is the most important. If I play too much, I get burned out, and play worse. Quality hours at the table are what matter the most. There's no point in playing if you aren't playing well.
So yeah, I need 2 days off per week. I don't think about poker that much when I'm off. I need to get away.
I would think each person differs. Only do what you are capable of, and strive to increase quality hours always. The end goal is to be a effin machine and play in the zone more.