As soon as you feel comfortable with. I started with kanji immediately when I started studying Japanese, because it opens the door to so much study material (lyrics, blogs, books, etc. -- it's faster and more fun when you can read them without having to check every single kanji in a dictionary). Of course, everyone has different study styles, but kanji are such a big part of the Japanese language and it will take such a long time to learn enough of them, that the sooner you start studying them the better.
My personal opinion? Start now. It IS possible to study vocabulary, grammar and kanji all at the same time. You can keep your focus on vocab and grammar, but seriously, the sooner you start learning kanji, the more you will thank yourself later.
Hint: Learning vocabulary written in kanji is a good way to learn different kind of compound readings etc. for kanji while you're also memorizing words. It'd be a pretty long and hard work to learn kanji separately from vocabulary, by trying to learn all the readings of the kanji without a word to associate it with.
Personally I have never learned kanji alone, it's just way too boring a drill. I've just learned them while I've been studying vocabulary, and I read almost as well as I speak and write.