Hombu requires even for yondan "only" 400 days.
600 days of practice for nidan seems a lot to me.
Yeah, really. Here are the requirements
from the USAF website. USAF requires 400 days for shodan, 700 days for sandan, and "if you have to ask, you can't afford it" for yondan.
Even the upper kyu ranks require enough hours that you're talking years, not months.