I for the first time have an understanding with my employer that I have about 5-6 useful hours in me. I am not someone who paces well and will go into a coding

I for the first time have an understanding with my employer that I have about 5-6 useful hours in me. I am not someone who paces well and will go into a coding frenzy until I drop, at which point I know I'm done. After that I go home and do other things (piano, study languages, etc.), but I couldn't write another line of decent code if I tried. Working a 30h workday with a typical salary is awesome, I have approximately the same output as I would working 60+ hours, leave the office during daylight and have time to explore other interests.