If the principal applicant and the derivative applicant were married before the filing of I-485s, and if both of them are present in the US, theoretically, their I-485 applications should get approved at the same time.

If they were married before the principal applicant's filing of I-485, but the other spouse is not present in the US, e.g., in China, then the principal applicant should file the Form i-824 with the USCIS concurrently with the filing of I-485;upon the approval of the I-485 and I-824, the file will be sent by the National Visa Center to Guangzhou Consulate Office, and the spouse can then apply for immigrant visa with Guang Zhou Consulate Office.

If applicant's approval of I-485 occurrs proir to the marriage, then this is a case of family based immigration. I-130 petition is needed, and the span of time for i-485 application is lengthy.