You make it sound as if there's a hard-and-fast deadline. In other words: "If Cassandra Cain goes a certain number of years without being featured in a new story, she will permanently slip away into Comic Book Limbo and
never be heard from again. It's a Natural Law of the universe. The only way for her to avoid this dread fate is for her to be recycled, even in a bad way, in one of the comics coming out from DC in the near future."
I simply don't see
why you are making such a sweeping assumption. Lots of DC characters have been in "Comic Book Limbo" for
decades at a time, doing nothing in any new stories (or maybe just showing up in a supporting role once in a blue moon), and then have made comebacks when someone at DC pitched an idea for a revival, and managed to get some editors to green-light his concept. It didn't matter how long the character had been offstage; it only mattered how much support there was for that character,
right then and there.
For instance, I once checked and found that J'onn J'onnz, aka The Martian Manhunter, a
founding member of the Silver Age JLA, appeared in just
2 issues of the original "Justice League of America" title during the entire decade of the 1970s (and a few appearances in other comic books of the 70s, but never as a "steady back-up feature" or "regular member of a team's active roster" in any comic book title of that era). Obviously, the League could get along just fine without him. For all practical purposes, it seems he'd been written off as redundant and irrelevant by the guys who were writing and editing the JLA book (and lots of other DC comic books)
at that time. But in the 1980s he made a successful comeback as a regular member of the Justice League roster, both in Detroit and then in the Giffen/DeMatteis era, and in the 1990s he had a solo monthly title for the first time in his career. His mostly skipping the 1970s didn't really hurt him any in the long run; there was no built-in "deadline" or "expiration date."
On a similar note, when Roy Thomas was writing his "All-Star Squadron" book in the 1980s (along with the "Infinity Inc." title and the later "Young All-Stars" book), he ended up dusting off a bunch of heroes and villains who had basically been languishing in Comic Book Limbo ever since the Golden Age. Some of them had probably gone over
30 years at a stretch without any new appearances, but they still looked pretty good when they were properly handled!