I live off my art, so I try to draw 8 hours for work, and then, if I have a free evening, I can spend like at least 4 hours working on my comic. I've been at this for the past half a year since I drew comic. And for most of the past half a year, I was working on another illustrating project, so I spent on average one weekend day working on that. And when I say day, most often I do mean from breakfast till sleep time.
So, I would say I draw a LOT. But, time spent doesn't equal improvement. I don't feel burnt out or anything, my comic is really fun for me, so I actually feel like continuing in the evening, but with my non-comic art, I feel like I've hit a plateau a few years ago and didn't improve much. However, since I never tried anything like a comic or drawing in that style before, I can feel the improvement. It may not be obvious for other people, but it is for me, and that keeps me motivating, too.
I would say you don't need to spend hours drawing a day. Even those 10 mins are fine, or a few hours after a few days of doing nothing. I think it is more about HOW you draw. If you actively learn and think about what you draw and how to draw it and practise effectively. I found for me this is really hard to do 8 hours a day. But when you spend the practise time on your own project, I think much less time is needed. You need the motivation. So basically draw when you feel like it and draw with the mindset to learn and improve