In the industry, artists keep 8 hour days and 40 hour weeks. So I'd say if you are drawing 8 hours a day, you are basically a full-time artist and doing what lots of people have done before you. However, most full time artists do take 5-10 min breaks every hour and they take an hour lunch. It's good to get up and move around. I suggest getting an artists table or do ergonomic things that lots of people do that have to work at computers or desks all day. As for your health, I suggest clean eating (whole foods, brain foods, avoid sugar and carbs, protein, etc.) and if you are going to snack, don't snack on energy draining things like cookies. (Just look up healthy snacks, google is your friend) Make sure you give yourself a weekend, or a day or two to recharge and just not draw to avoid burnout.
I worked full time and put myself through grad school, so I know how it feels.
Although my full time job wasn't art, I was perfectly happy to put several hours a day into my webcomic.
I'm a mom now, so I can't dedicate that kind of time to art like I used to, but I like to cheer on those who can. 