A recess for making appointments must be at least 5 days long. Obama got burnt on that a couple times and it went to court....and they shoved it to him.
The 113th Congress ended session 2 on December 16, 2014, and formally adjourned on January 2 2015. The 115th Congress began session 1 on January 6, 2015.
The "recess" that tripped up Obama was the Article I Section 5 "Neither House, during the Session of Congress, shall, without the Consent of the other, adjourn for more than three days, nor to any other Place than that in which the two Houses shall be sitting."
Both chambers must agree within 3 days to adjourn, but this doesn't end Congress. They can refuse to adjourn and continue to gavel in a pro forma session every three days. There is nobody to do this in the gap between Congresses, so Obama will have 4-6 days of no Congress at all to make a recess appointment.
-PJ