Obama isn’t going to use the recess option because the nomination would be wasted. The position would end up being vacated at the end of the year when the nominee was not approved. He wants to change the court complexion for as long as possible. He will nominate a young person.
A recess appointment must be approved by the Senate by the end of the next session of Congress, or the position becomes vacant again; in current practice this means that a recess appointment must be approved by roughly the end of the calendar year.