回複：I-485 was updated again to Post Decision from Decison. Why??

I got the welcome notice by mail last Saturday. Today I got the email it was updated to post decision from decision, but not my son's case. What does this mean?
-It just the proof of your I-485 approval.

I went to infopass to have the stamp last week, yet I was told not ready yet.

Does this mean I can go now to have it?
-Yes. Bring the Welcome Notice with you.

How long it takes to get the green card?
-A few weeks to two months.