Iain McCaig says an hour a day is enough, but then again he's Iain McCaig and most of us aren't.

I would (and do) approach improving in art the way you'd approach getting in really good shape. Ease into it. Don't start drawing 10 hours a day from week one. Start with something small and challenging, like those 20 minute Croquis Cafe gesture videos on youtube. Do one a day for a few weeks.

Then work up to the New Masters Academy 35 minute videos. Again, once a day for a few weeks.

A month later, add some more weight to the bar. After doing the "warm up" gesture drawing videos, add a 1-2 hour long study of anatomy or drapery every day (or at least 4-5 days a week).

A few months after that, add another hour or perspective drawing.

And after that, add an hour of drawing from your imagination (which is hard, but will be somewhat easier after spending months doing anatomy, drapery and composition/perspective studies).

On the weekends, if you really want to challenge yourself, spend some time doing master studies/copies of art you love. Doesn't matter what medium you choose, just do it without tracing.

Once you establish a routine and learn how to study the fundamentals, it's just a matter of grinding it out until your work looks like the type of work you see on Artstation or other pro-level art sites.

When you feel like you're stuck or on the verge of burning out, take a few days (or a week) off.

Don't become a hermit, dedicate time for family, friends and other hobbies.

Try to soak up as much info as you can from other artists but be aware that many of the pro's out there love using clickbait titles and will basically rehash things you already know (similar to bodybuilding magazines that have titles like "Get a Beach Bod in 1 Month!").

Art takes time and is frustrating for some people (I'm always frustrated but still enjoy the process). What separates the pros from the amateurs isn't talent, it's dedication, hard work and constantly adding weights to the bar and doing things that are outside your comfort zone - which is the only way to expand your skill set.