Kateko
By the time one starts learning Japanese as a second language, they're generally way behind any native Japanese student's kanji education (obviously).
It's something like 2000 kanji before you'll be able to read a newspaper. That's going to take a good amount of time.
A surprising amount of people learning Japanese don't seem to really think learning kanji is worth the effort, but...you're not really learning Japanese if you can't READ it. So I'd say start asap.
Obviously, though, you need to be learning the rest of Japanese alongside it...
I'm afraid I have to disagree here. Languages and writing systems are two separate things. Language is used to communicate, while a writing system is used to record language.
As for the main question in this thread, for most languages this is very easy: Simply learn the writing system alongside the language itself. The reason is also simple: Most writing systems are easy to learn, because the characters in them don't mean anything, they're just pronunciations, and the number of characters is limited.
With Japanese, the above only applies to kana. Learning this is like learning the Greek or Russian character set, so learning kana is pretty straightforward. However, written Japanese makes extensive use of a subset of the Chinese character set (I think it's 6355 out of 40000+ chars, with 1945 kanji for general use), and the problem with learning kanji is the amount of things you have to memorize for each kanji: 1. The readings 2. the meanings 3. the character itself. Usually there are only a few readings you have to learn, but if you look at the wash list of meanings (take a look at Jim Breens kanjidic file), then it becomes clear this is no quick task.
The problem is that when you don't know Japanese very well, you'll often need to memorize the kanji's meanings
and readings, while if you know Japanese well, you don't have to look at the meanings at all! So I think it's better to learn Japanese and kana first, and start learning kanji when you've built up a good enough vocabulary to understand most spoken Japanese.
By the way, I'm not a Japanese learner yet! I've been interested in the Japanese writing system for some time, and I'm currently working on some dictionary/learners software, which is the only reason I know anything about written Japanese in the first place! However, I decided to post here anyway, because I feel my opinion seems to make some sense.