[...] I would be curious to know how you go about tackling them. I remember you telling me in the past that you had a great sense of memory, which is definitely the most helpful ability one can hand to tackle so many games in such a short amount of time, but beyond that, how do you do it?
When I started out, I would only play them on a console without any means of efficient practice, I also didn't watch any videos save for a handful of exceptions. I believe that helped quite a bit in learning about various types of classic shmups, even those with awkward hitboxes and whatnot. I made a point out of learning something from every game, even if it is some horrendous Dimension Force or Xenon 2 Megablast. These days, it depends on the game. Hacha Mecha Fighter for example I played two afternoons before we were talking about it here, without any pressure or intention of clearing it. Once it became lucid that I would like to beat it I decided to immediately use the most efficacious tools available for I knew that the game is almost entirely based on memorization/planning. Watched the PCB run on YT which gave me a general idea for everything, then set up savestates to look where I also would have to bomb or perhaps could play in a slightly different manner.
Most games I like to play more natural, though, without immediately resorting to savestates/videos. I would imagine that due to the fact that I've seen a lot, I tend to find viable (not necessarily ideal) solutions fairly quickly most of the time. I also simply enjoy dabbling with a lot of games, even with their higher loops. Having said that, something like the second loop of Image Fight, R-Type II and Xexex I would not even dare trying without proper practice.
[...] what kind of time commitment do you expect is necessary for a relatively hard shmup (say, in the 25-30 range)?
That really depends on what type of difficulty we're speaking of. It only took me a handful of hours (five hours spread out over two afternoons) to 2-ALL X-Multiply since it is so inclined on memorization, while Thunder Dragon 2's fast bullets thwarted more than one attempt (I think I've had to spend about thrice that time). Consistently clearing, say, Gradius III is definitely much more realistic for me than something with frantic gameplay.
what about the toughest clears you've done? I'm thinking in terms of hours rather than days or weeks, BTW.
If we're talking about all of my clears, including marathon runs, then definitely my Gradius II 5-ALL, with the Gradius 8-ALL coming fairly close. The first two arcade Gradius games (and probably Gradius III SFC) are the games I've played the most over the course of the years, they were also among the first games I've got at around 2010. Most assuredly dozens upon dozens of hours for all of them. I have the feeling that - in terms of a 1-ALL - Gokujou Parodius with the special stage will be the game that took me the longest once/if I finally manage to beat it, that special stage alone took (still takes me!) an ungodly amount of time, can't get consistent at it.
Also how long have you been playing Shmups? Is it right to assume you were far from a beginner by the time you signed up?
I've always liked the genre, but only played them in a casual manner back in the day. Around 2010 I got into 1CCs and all that, by the time I've registered here I've worked my way through most 16-bit shmups (started with the SFC, then worked myself through the PCE and MD) and a few NES/PS1/PSP ports/console-exclusives, so I had a bit over a couple hundred clears when I registered.
Edit: added remark for Lethal Thunder (-25 compared to Thunder Blaster).