Hombu requires even for yondan "only" 400 days.
600 days of practice for nidan seems a lot to me.
A black belt at hombu means something different than it does in the states. At Hombu they receive shodan around the same time we receive 3rd kyu in the states. To the western world black belt has a connotation of "mastery" , thus the standards for many western federations are higher for black belt. In Hombu a black belt simply means you are a contributing member of the dojo. You are considered to be a committed member of the dojo there around the same training time it takes the western federations to hit 3rd or 2nd kyu.