Replies

The deadline is that they have to be appointed in time to vote on December 19.
(But note the “any controversy ...six days prior” language.

“On the first Monday after the second Wednesday in December (December 19, 2016), the electors meet in their respective States. Federal law does not permit the States to choose an alternate date for the meeting of Electors—it must be held on December 19, 2016. The State legislature may designate where in the State the meeting will take place, usually in the State capital. At this meeting, the Electors cast their votes for President and Vice President.
If any Electors are unable to carry out their duties on the day of the Electoral College meeting, the laws of each State would govern the method for filling vacancies. Any controversy or contest concerning the appointment of Electors must be decided under State law at least six days prior to the meeting of the electors. “
https://www.archives.gov/federal-register/electoral-college/roles.html