Lithium depends on blood level, not dose. So it depends on what your level is. Have you had a level scheduled? Usually they can do one about 4 days into starting or changing the level. But until they do a level you won't know if you are on enough or too much or too little. I've had a level of 1.2 on 400 mg or on 1200 mg depending. 900 is a moderate dose but it really depends on how you metabolize it.