UPDATED 10/26
As things stand right now...
At present, 17 Cubs minor leaguers are eligible to be MLB Rule 55 minor league free-agents (6YFA) at 5 PM Eastern on the 5th day following the final game of the 2018 World Series, and 67 Cubs minor leaguers (including five who were eligible to be minor league free-agents post-2018 but who signed 2019 minor league successor contracts) are eligible for selection in the December 2018 Rule 5 Draft.
So the Cubs have until five days after the final game of the World Series to either sign a post-2018 Rule 55 minor league free-agent to a 2019 minor league successor contract or add the player to their MLB 40-man roster, otherwise the player will automatically be declared a free-agent.