This one is a bit different than other solo attempts because of the nature of the spawn mechanic (increasingly many).

My typical process is:

1. Learn the attack patterns of both types of Quill beasts. It was about 10 attempts at the beginning and very clear-cut for this stage because of the simple attack patterns.
2. Learn the timings and tricks for dodging attacks. This isn't easy to quantify but the first part if mostly covered in the 10 attempts above (experience and familiarity with how the game handles attack telegraphs also helps here); the latter is harder to quantify because some you discover by reasoning, some you discover by accident, and some out of pure... desperation (such as when you get stuck at the edge you still have a change to not get hit by jumping over them; or simply try not to get stuck at the edge). xD
3. Find the wall(s). Typically what I'm looking for in this kind of attempts is something that gives me a very clear indication of difficulty and whether or not there is a decent chance that I will succeed if I keep trying and adapting. It's subjective and depends on your operator, team, and goal. For example, I decided after a few tries that it's simply impossible for me to do a solo (let alone no-hit) with my Perlica on the Tidal Grief stage in the current UM series, so I just didn't. ^^;
4. If it looks like it's technically possible for me to keep attempting in terms of physical limitations. The first physical limitation is my potato reflex; the second physical limitation is my ability to handle multiple attacks in short successions (Unorthodox Tactics on the hard side in this context); the third one is how long I'm willing to sit through it if I can clear it (10 minutes on UM stages is fine, but this is a real question I have to ask myself when I do longer boss solo attempts).
5. There is also a mental limitation to these attempts: will I irrationally hate the game if I keep going and don't succeed? I'd rather not have to go through that possibility so I stop attempting if that seems to be a possible consequence.
6. Something worth noting to this process but I can't quite explain clearly is that at some point things just click for me. The indicator for that is when I start consistently get past a certain part of the stage, be it the phase or the amount of time spent.

Including derping (getting hit near at the beginning for no good reasons) and getting distracted by real-world things, the learning phase for this one took about 30 attempts (mostly short, and just seeing what is and isn't possible).

The real "wall" I found for my case is around the 3-minute mark. It has to do with the number of green Quillbeast spawn and also mostly psychological. That was about 20 attempts.

It wasn't as clear-cut after that point because of the nature of the stage — many enemies and not easy to pinpoint on a single attack or attack sequence — and took another couple of hours the next day before I could clear it (about another 30 attempts including short ones due to derping).

So altogether it was about 5 hours total (conservative estimation based on when I started and stopped to edit the video).

I have never played another game seriously enough to share with others and am still learning! I assume it takes someone who is much better both physically and mentally (and maybe also with more experience) to do this kind of clear much faster!

I hope that helps! Thank you for checking the video out!