8-10 hours with few breaks. (I used to skip breakfast and sometimes lunch too...coffee diet) It started as a necessity and then became a habit. I have to retrain myself to take breaks and walk away and give my brain some breathing room and fresh air. This doesn't include the time I spent thinking about problems while driving or in the shower.... I've made progress breaking those habits though.
I can relate to this. It takes quite a lot of effort/mindfulness or cannabis (or all of the above) to come down off the work buzz and actually stop working in my head! Otherwise, as much of my day as I can get away with will be either work or personal projects (which are very similar to my work for the most part)