It’s surprisingly doable. If you have a free weekend you can learn all of them. Even one day may be enough if you are dedicated and a quick learner. Of course, repetition is a must if you don’t want to forget them again, but the initial investment is really not that big.
This resource suggests mnemonics which are surprisingly effective. They teach hiragana first, but it’s not a hard requirement if you want to learn katakana exclusively.