It took me about 5 months in total, from the day I picked up the application till the day I got my HKSAR Passport.
You don't need to go through a lawyer. The process is simple. Fill in all the forms and submit required documents. They will send you a letter saying its under process. Few months later they will call you up for a quick little interview and ask for further documents if required.
Few weeks after that, they will send another letter telling you if your application is successfull, to surrender your current nationality and bring that surrender letter to complete your Chinese naturalization.