What's the period time to receive approval from USCIS for a green card?

There is no single time period for USCIS approval of applications. The USCIS is inconsistent in the amount of time it takes to adjudicate cases, and its processing time can depend upon such issues as the type of case (marriage-based adjustment of status, other family-based adjustment of status, employment-based adjustment of status, applicable USCIS processing center, absence or existence of a visa backlog, quality with which an application has been prepared and thoroughness with which supporting documentation has been supplied, complications regarding security background checks, etc., etc. An immigration attorney who is thoroughly familiar with your case would be able to give you advice about ordinary processing times and expectations for your case.