I've done that with Russian, got around level B2 so far. But I had quite a bit of experience with language learning before that (mainly easy-ish languages: Spanish, Portuguese, German, etc).

Russian is tough nut to crack, and it does require above average dedication. As a matter of fact, I'm currently in Kyrgyzstan for a few weeks to get some immersion and hopefully break out of the level I'm currently in.

You'll need loads of time, but for some languages you really do need to find grammar interesting.