If you work 12 hour days, then you should only be working 3 days a week. If you can't make it work, then work out on your off days.
I recently started working 10 hour days, but have a 30 minute commute each way. I get up at 5 am and workout for 20-40 minutes (less on hair days, more on non-hair washing days). I also do 20-30 minutes of weight training when I get home from work before dinner. I try to get 10k steps in before the end of the work day. I try to stand as much as possible and just simply try to keep moving which is hard when you have to sit and chart.
I recently changed jobs which is why I am now working the longer days. I have noticed that I am not as active at my current job and have found that I have struggled to lose weight. Now that I realize I am not burning as many calories, I am planning to cut out an extra 100-200 calories on work days if I don't starve
It takes some serious planning. Clothes are put out the night before (or if I am super motivated, they are put out on the weekend for the entire week). Clothes for work are also put out. Lunch is prepared ahead of time on the weekend as much as possible. I also cook on the weekends and that allows me to just warm up dinner w/ little to no prep. All this helps me make time for the workout. I really want this, so I make it work.