Well said, though I do respectfully disagree with one thing: "Take you no more than a couple hours."
The computer I had been playing this on for a couple of years bit the dust (simultaneous motherboard & HDD failure) a couple of months ago, so I started fresh on my new replacement computer. Maybe I'm doing something wrong, but I've spent about 5 game hours or so playing HI, and I would estimate that I'm a little less than half way to where I was before (e.g., no Mercedes/Miriam or Yumiko yet, but they should be arriving soon). I don't perfectly optimize absolutely every half-hour of gameplay, but I think I'm reasonably efficient, and I generally play with the intent of maximizing my enjoyment, so it's all good. I've also avoided many of the mistakes I made the first time, thanks to the benefit of experience and more careful reading of the guide (many thanks for that, btw).
Bottom line: Someone with your level of experience and familiarity with HI, who was strictly following a critical-path optimization strategy, could probably complete all of the achievements available to date in a couple of hours or so, but us mere mortals will likely take much longer than that (and that's not necessarily a bad thing).