How many hours are you productive per week and per day ? And how many hours at a stretch?
For me - I have difficulty doing it more than 20-24 hours a week on an average. I'm fast at coding and problem solving - but that also means my energy wanes off faster within the first 4-5 hours and I am better off not working the rest of the time. Just in case you are wondering - I never had any problems related to productivity at my 40hr/week job except that I had to endure the additional frustrating hours.