How long does it take to get Chinese citizenship by descent

My father is Chinese, lived in Thailand more than 42 years until he passed away, I was born in Thailand, 42 yrs old, but I want to apply for Chinese citizenship, I am willing to renouce Thai citizenship, a female employee of Chinese embassy in BKK told me that application for Chinese citizenship in this case isn't possible. Please help, if it's feasible, how long does it take for process and citizenship granting