3-4 solid hours of productivity is my sweet spot as well. Though it took me a while to figure it out. There are times when I have pushed 6-8 productive hours

3-4 solid hours of productivity is my sweet spot as well. Though it took me a while to figure it out. There are times when I have pushed 6-8 productive hours per day for 2-3 weeks but what usually follows is 2-3 weeks of "unproductivity". Over the long run this is unsustainable and has caused me to switch jobs in the past.