Well, to start with, kana usually doesn't take much longer than a couple weeks of repeated exposure and practice, especially if you learn to write them. It's not much harder than learning the alphabet. So I'm gonna move away from hiragana and katakana because people usually mean kanji when they talk about this.
So, personally, I think defining it in terms of "study time" is slightly around the issue, because in my experience it's less about how much time you spend reading textbooks or listening to exercises and more about how much you put what you're learning to active use - using it in a way that will engage you, and actually make you excited/satisfied with what you've managed to achieve. You know how it's usually easier to remember something if you write it down in a note for yourself? It's kinda like that, I guess. But to answer the question, I learned kana like three years ago, then put off studying kanji until about a year ago, if I remember correctly. I'm nowhere near fluent, but I can read a pretty good range of basic sentences on sight. But I'm also not one to push myself very hard (been treating it as a hobby mostly), so I imagine a lot of people could learn much faster. I know people get n3 or n2 certifications within a year of consistent work.
But yeah, cramming lacks association, so it's more prone to slipping from your memory. I really believe you gotta "use it or lose it" if you want what you're learning to stick. If you have strong associations, it forms a kind of memory chain (this all must sound very scientific...). I've been learning kanji with wanikani, and some words they teach me take longer to memorize if I've never heard them before or never had to connect them to anything in the past. As an example, in one of the first chapters of Ueno-san wa Bukiyou I ever translated, they kept talking about 匂い - nioi, or the noun "smell." That word is drilled into my mind now, because it'll always remind me of that one chapter. Wanikani hasn't taught me that kanji yet, but I already know I'll never forget it. So in some ways, trying to learn by connecting it to reading manga could help in the long term. Don't be afraid to jump into trying to read some raw manga even if you think you're not ready yet, because it could add some fun to how you learn certain things.
And the funny thing about your goal - that's another helpful thing to keep in mind, by the way, is making your goals as specific as possible - is that "reading" manga can boil down to simply learning what the kanji look like. You may not have to practice writing them or bother memorizing any readings/pronunciations if all you want to know is their meaning. Scanning a raw manga page and sightreading it can be as simple as knowing the definitions even if you wouldn't be able to "read it aloud" in your head. But obviously, that's not the same as learning Japanese to the point of fluency.
Anyway, didn't mean to ramble. TLDR: cramming info is inevitable when studying anything, but finding ways to make that knowledge useful to you is how it'll stick.