To start with, it really depends on what your core reason for drawing is.
There is seriously no law you have to follow. There is only the guiding principle that, yes if you do any kind of persistent and consistent practice, you will improve, regardless of the skill you're working on.
I agree that even a 10 min sketch practice counts. Just know that it won't hold the same amount of weight as an hour or two hours of a class style figure drawing session. You can still 'get to where other artists are', it'll just take longer.
For argument 's sake say you're starting out and are a beginning level artist, right?
If your core reason for drawing is for the fun of it, and making art and comics is a hobby, then you, and only you decide how much you would like to draw each day. There are no right or wrong amounts there. Hobbies are meant to add flavor and enjoyment to life imo, so there's no need to compare to other artists. And why would you? It's just a hobby.
Now say your core reason is to seriously improve and not take forever in doing so, whether for yourself or because it's key to a career you want to have in comics. (I am of the latter persuasion.)
Then I would say drawing every day for at least an hour (not counting days where breaks are necessitated for good mental and physical health) is vital. I say Vital and not required cause there are no rules, only tried and proven principles of work that will help you improve the fastest.
But you should also be smart about how you practice and what you practice. Focused and deliberate practice means having the goal to learn something each time you settle in to 'train'.
When I first started drawing, you couldn't KEEP me away. I loved everything about drawing. I started seriously when I was ten and I have sketchbooks chronicling my progress since I was twelve.
I've now been drawing for 15 years, and 8 of those years are professionally. First in freelance commissions and in the last year, working in partnership with Tapas for the Incubator Program. (My first breakthrough after 5-6 years of seriously getting into the field of comics)
This is what my experience was: I was mostly self motivated in drawing before college. And I really just loved to do it so I drew on average 1-4 hours each day when schoolwork didn't get in the way (and even when it did haha).
When I started my Animation degree in college, I took three studio drawing classes over three semesters: Drawing 101, 102, and 201 for intermediate drawing so I would get the sheer hours of deliberate practice in.
Classes would be three hours, 2-3 times a week for the whole semester. That came to dozens of hours each week (counting homework practice) for 'training'. At the same time I was still doing online commissions. Each day had an hour, two hours, of creating and applying what I'd been training and learning.
During that time I was easily studying art for 4-5 hours a day and applying the knowledge for 2-3 hours more. And it got to be even more drawing when I started the Animation classes.
This is an admittedly extreme side of the work spectrum of artists lol. There is NO reason to do this much unless your really, really want to. Everyone has their own path and you should decide on the path that best suits your goals.
After finishing college, in order to make any kind of income, I worked on drawing and learning better ways to sell art for the same amount of time, and more, than a full time job. So say 6-8 hours drawing time a day. I created more than I trained in art so sometimes I plateaued if I wasn't actively trying to learn something new.
Plateauing is normal for every artist. It happens when you're creating more output rather than input drawings.
Finally to answer your question: I don't study at all for the time being. I draw a minimum of 6-8 hours a day and it all goes into the production of my incoming story, Swaha. Weekends sees a fluctuation of 3 to 8 hours If I'm not properly resting my hands instead.
Once Swaha is done, I plan to shift to a 50/50 training and producing schedule of art so I can learn while I'm making stuff for my other comics. It'll still probably be 6-8 hours of drawing on weekdays, now simply because I've made a habit.
Getting better at drawing, or really any skill, is simple because hundreds of others have walked those paths and shown a myriad of ways to get to their level. However its important to know that though it may be simple it's not easy.
You just have to decide for yourself how much drawing will fill you up in a given day. If it's ten minutes or eight hours doesn't matter. Just be clear about why you're drawing, and that will give you the motivation to draw exactly as much as will make you feel good.