If you already learned hiragana, then it should be much easier to learn katakana, because many of the characters are similar. However, how you learn it and how long it took you to learn hiragana both play a factor in how long it'll take you to learn katakana. It took me one week, because I aimed to learn 5-10 a day. I think you can learn it faster, but I'm sure it'll take at least a few days. I used a book so that there was space for me to practice writing it. Good luck.

It took me one week to learn Hiragana (+ dakuten and hando, combo hiragana) and a day to learn all of katakana (+dakuten and hando, combo katakana)

Keep on going until you have memorised them all!

You've memorized hiragana already, so katakana shouldn't be difficult, now that you're used to the symbols. Some say it can take just a weekend if you push hard at it. It depends on how many you aim to learn a day. You'll learn it quite fast, I'm sure. I learnt the hiragana quite fast, but, funnily enough, it took me longer to learn the katakana. Yeah, it depends on you yourself.

I understood it as Katakana, and once you get to know Hiragana, I think it won't take you so long to get it. because they have same rules and pronunciations but different characters (still many of them are similar to Hiragana and even simpler.) がんばって＾＾