@rtyu: There is no time frame, they do it whenever they feel like it. It's only been a thing for a couple years with no set pattern.
OT: The most likely occurrence would be when they implement the patch that merges cs inventories of characters in the same branch like the Resistance(similar to how it is with Explorers). This is all assuming Nexon NA even goes that route.
@rtyu: This has been by far the longest period since it's introduction that we haven't had the event. While it's very likely that we'll get one before the year ends, since the major updates and events occur during the holiday seasons, we didn't get one this Spring, Summer or Fall despite constant speculation so it's entirely possible that we may not get another one at all this year.
Also, when did the 2nd one happen, I only recall the one in March.