One. One day of intensive study is all you need. Of course most people spread it out over a few days or weeks. Really locking them in could take months or years.
You can learn it in as little as a day but I wouldn't say you should, regardless of how long it takes to "learn" them all it will take at least a few weeks to have them memorized completely usually, and longer to know them by heart. Generally people take a week or two to learn them, but you shouldn't worry too much about how long it takes anyway. You have to make sure you go at your own pace or you will learn them badly and then mess up your pace later.
As a point of reference, it took me about a month (~4 weeks) to start knowing what sounds all of the characters represent at just a glance (2 weeks for Hiragana and 2 weeks for Katakana). It’ll likely be different for you and how much time you put into it. You’ll only get better with more time spent studying this language.
Worrying about how long it takes is inconsequential. It's not measurable in the sense of the idea there's some sort of boundary to it. It's not important. Same goes to the rest of Japanese. Despite that it's one of the most straightforward subjects of the language, it's entirely subjective to the student.
Of course, you'd be using it all the time once you stufy the rest of Japanese. But, give yourself more time to know it before you learn the rest of Japanese to avoid the early stalls and subsequent regrets.
I learned it in a month, because a person needs interval repetition so that it really sticks in his head. The goal is not to memorize everything in one day, but to remember it forever. You should always choose to study every day, but little by little.
Anyway, if you need to hurry up, I think you can learn faster than me. I hope you can achieve your goals, good luck!
For me, learning either was rather quick. To learn anything, you need repeated (and meaningful!) exposure to it. You could learn hiragana and katakana in a day, though your retention of it the next day is quite low- and a week out you're probably even lower. I found mnemonics to be helpful for my first few days, and then just writing them over and over again (lol). I learned (as in able to recognize and pronounce) in about 3-4 days, mostly in day 1 and 2 with day 3 being a review of what I remembered and so on. After a week I was reading hiragana and katakana as if I was reading English letters, and generated my own ways of remembering each character. This also happened because I was really set on learning Japanese- I spent a majority of my free time trying to learn. That's not applicable to all learners, so repeated exposure over the course of a few weeks is likely how long it would take, even with 15 minutes a day. There are (free!) apps that help with this, which you could check out.
Side note, I suggest finding ways of remembering them that make sense to you- sometimes I made my own mnemonics and other times, especially when learning katakana, I would look for similarities (or what *I* considered similarities) in their hiragana counterparts- you'd be surprised how *some* match up. But this doesn't work for all of them (trust me! サ looks like せ but they are different sounds!).
How long should it take? Depends on many factors impacting your learning, most of which we don't know. Plus, "should take" is irrelevant. So, just take it step by step. Repeated exposure is key! But you needn't worry about where you should be- learning is a process. Some people pick things up quickly, others take more time. It isn't a reflection of your ambition, intelligence, willingness, or any other factor- your biggest motivation should be you and your biggest competitor yourself. Get better slowly, everyday, and you'll be flying through hiragana and katakana in no time!
It didnt take much for me, probably at most 5 days. If you struggle with remembering kana, op and commenters, you should try Tofugu's learn hiragana & katakana. It is a webpage with mnemonics (little stories) for the hira and kata + it has mnemonics for the tenten (ば) and maru (ぱ) versions, and the other strange katakana you see like ヴぃ. It has a built in test divided on batchs for the readings once you study them, and the website itself is pretty good too! Hope it helps
