How hard is it to learn Japanese?

How long does it take? How do you memorize thousands of kanji? I've always wanted to learn Japanese but whenever I start it it ends up petering out after few months and I go so long without studying that I forget a lot of it.

I've been teaching myself Japanese for several months now, and I've made good progress.  You should start to learn the ひらがな (Hiragana) and カタカナ (Katakana) first.  It doesn't take much time to pick them up.  I made flash cards and I was able to read and recognize both systems in one week.

The grammar is also a lot more logical than English in my opinion.  You should check out http://www.humanjapanese.com, it's a great starting point. 

The Kanji certainly takes quite a bit to pick up, but it's something that you should focus on a little later down the road.  This is a good book that I've been using:
http://www.amazon.com/Essential-Kanji-Characters-Systematically-Reference/dp/0834802228

I usually make 10 flashcards at a time for Kanji.  After I understand the reading, pronunciation, and if I can write them properly, then I move on to 10 more Kanji characters.

Good luck!