Timeline:

4/20/2014: application sent out!

4/28/2014: Fingerprint fee is accepted

5/20/2014: Papa's fingerprints taken

5/29/2014: called USCIS, was told Mama's appointment letter lost, can go to do the fingerprint at any time, no appointment was needed!

6/10/2014: Mama's fingerprints taken

8/4/2014: no need for interview mail

4/8/2015 welcome letter received, card will be received in 3 weeks!