I’ve never taken the JLPT at any level before, but I know a friend who took it as a subject at school, and he got the N1 six years after starting. The class schedule was roughly 3h/week for the first four years, and 4h/week for the last two. It’s definitely possible to learn faster, but I guess this proves that 2 years isn’t entirely unreasonable? (But ok, I’m being really lenient here: dividing six years by five levels gives you ~1.17 years/level, which isn’t accurate because the higher levels require more time.)
You’ll probably want to look at the class hours and what’s being studied (you could check the textbook that will be used, for instance). Talk to people who have taken classes with them before if you can. It’s possible that they created the two-year time frame based on the assumption that their students wouldn’t necessarily do much work outside of class. I can imagine it taking 2 years in that case (even if I agree that it’s probably too long).
Another thing: keep in mind that the JLPT doesn’t test everything, and even people at N1 can have huge holes in their knowledge. (Anything that’s formal Japanese should be fine, but they may not have knowledge of slang, and there’s no guarantee that they can understand specialist topics that might come up in Japanese university lectures right off the bat. The JLPT also doesn’t require full spoken or written fluency because it’s multiple choice.) The friend I mentioned himself said even the N1 means nothing beyond the sense of achievement it gives and its practical utility (e.g. landing jobs in Japan or applying for a course at a Japanese university). Getting to N5 by focusing on the JLPT syllabus alone definitely shouldn’t take 24 months, even at the rate of one lesson a week, unless students do no work outside of class whatsoever. However, if the school you’re looking at aims to get you to N5, but offers a lot of ‘value-add’ on the side (e.g. cultural stuff and knowledge that’s useful but not required to pass the JLPT) then ok, maybe it’s still worth it. Depends on your goals and preferences.