Rybka is better than top-20, so I did not claim that.
I was a half-pro ~10 years ago. I won the championship of my country. I wrote an M.Sc. thesis on evaluation tuning. If I'd use the standard approach that I already know, 6 months is a conservative estimate. The question is - what for?
Your comment seems either ignorant of the amount of work involved, or very arrogant about your skill, but does not provide any information for me to judge.
Why do you think you could beat even rybka with just six months of work?