It doesn't take more than 2~3 months if you have good background in Biology, which takes the most time memorizing and practicing problems.
It took me 5 months to complete Gchem and Ochem destroyer section due to school/work but I'm sure people can finish them much faster if they study 3~5 hours per day solely on DAT. I have 2 other pre-dental student friends in my school, we all used Chad's Video and DAT destroyer and all 3 of us scored 20+ on chem sections, so I could say they are must have for studying materials. They will help excel in your orgo exam too if your professor is bad at teaching. Most of my difficult orgo exam and lab questions(in school) came from Chad's video or Destroyer.
I think 9 months of studying could be physically and spiritually too much for most students and kinda slack off half way.
But everyone's different, do what works you.