It depends how often your parents have been in USA. In my mom's case, she
has been in USA for more than 2.5 years in total (coming back and forth between usa and china) for the most recent 5 years. SO this
time after she stayed in usa for 1 year, applied visa through Zhongxin after
she has been back for 6 months, but didn't get it. She is asked to go to
interview, and still didn't get it. the officer told her to come back for
interview after staying in china for 12 months. Hope this helps!