Hmm, I dunno mate, when I hear specific details like that I wonder where the source of data comes from. My personal logic, my opinion, is that everyone is different and some people will learn better one way others another - also, why limit to one study if you can manage more/ assuming the difficulty is appropriate for one's level and that you are trying to accomplish a clear goal within your curriculum.
For example, maybe a fully rendered portrait that takes four hours, you know, realistically you'd only do one of those in a day, or you would do a little of that for a few days until you're done to have more time for other things. Whereas, if you are doing quick 10 min block ins of heads, you could easily do 3 to 6 in a day if you have the time and energy, and mix it up with something else. All depends on you in this case, as the beauty of this kind of course is you can take it on your own pace.
When I have the time and energy to dedicate myself to art in a day, I've easily clocked 8 hours sometimes in a day XD but I am an adult and have adult life stuff to contend with, so that's not usually possible - heck for many people it's also probably not recommended either XD
One thing I note on your last piece, you can take a little longer there and try to refine your 3D forms, try adding a layer for the refined work and try making some line weight decisions, take your time to make corners, some edges, intersections darker/weightier, and things going back faded and thinner. Avoid tangents (overlapping lines that are parallel and merge into each other like the contour of the cylinder on the far right. This is a great opportunity to do this, and honestly even I should have and should still do that more often, it really gets your lineart to the next level as you progress. Just look at @Pattu_w 's work for inspiration, if there's anyone I've seen do this really well it's him, from the 3D forms now to portrait lineart, you can see how one thing leads to another. In other words, don't underestimate the simple exercises
cheers!
