What is the learning curve like for learning Japanese writing?
About the same as English. Chances are you didn't start learning to read English by pedaling your five-speed Schwinn (with the baseball card in the back tire) to the local library and checking out Pride and Prejudice with your shiny new card. You had to start with the Easy Readers, wherein you discovered, I hope, Dr. Seuss stories with all those wonderfully rhyming monosyllables. Along the way you learned that the letter C is a mystical letter of many sounds, and someone had to tell you that the Cs in cake, cell, and cello all sound different. You also learned about heteronyms, those quizzical words which have identical spellings and different meanings. Eventually you reached the point where you can pick up the latest New York Times and understand every sentence (even if you don't understand the story behind the sentences).
Japanese is no different. You start with the kana and gradually tunnel through Kanji Mountain. As you go you pick up on the rules of the game: this kanji is pronounced this way when it's part of a compound and that way when it's alone, or this compound looks like an onyomi compound but it's really kunyomi, and so on. For a long time the dictionary is your closest companion, but the longer you study the more your reading range expands, and the less you need to rely on your trusty indexed friend. If you stick with it, you'll eventually be able to comfortably read a letter from your electric service provider or browse the new fiction in the bookstore without immediately needing a dictionary. It takes years of study, but so did English, and it's highly rewarding.
Are you able to use what you've learnt as you're progressing, or are you only realistically able to start reading real text once you've learnt about two or three thousand kanji?
What is "real text" to you? A newspaper? A Murakami novel? A graduation thesis? If you restrict yourself to these "grown-up" materials, you will likely end in frustration. Nothing is stopping you from reading anything, but the lower your reading ability, the more of a grind it will be to slog through the unfamiliar kanji and grammar in high-level texts. Progress always depends on studying things that are just above your level. If you're working on grade school kanji, read material targeted at grade schoolers: the kids' section of newspapers, manga for kids, and so on. (This is why I personally recommend studying kanji in the same order used in Japan.) When the material you're reading becomes too easy, move on to something more difficult. It's all "real text", and everything is a stepping stone to get you to what you really want to read.