We can share the minimum amount of days of inactivity that must have been passed. Please keep in mind that it could always take longer (not shorter) to get moved, though (e.g. if it would take a minimum of 3 inactive days, it could also happen that the movement would take place only after 5 or 6 days if any "blockers" occur. The aim is to strike a balance between no interruption and a fast as possible movement. These are the minimum amounts of time needed:
Chapter 1 AND Non Premium User : 3 days
Chapter 2 AND Non Premium User : 14 days
Chapter 3 AND Non Premium User : 30 days
Chapter 4+ OR Premium User : 60 days
A "Premium User" means someone who has purchased diamonds at least once.
From the discussion topic of the above link.
Those 60 days means that player should not have logged in even once in that period, just because there is 0 activity if that player even slighty peeks at the (former) city every some odd weeks it will never be put into storage.