In my case my ex talked about, not so often but not so rarely, her most important ex bfs.
I think it depends on the individual, but I guess it's safe to say they think about us often, even if at an "internal rumination" level only (so, it's also possible that you'll never know).
Fact is, they create so many traumas on themselves and on their ex significant others -- and they try to avoid processing these traumas with every bit of strenght in their soul -- that I'd be suprised if such traumas would not re-emerge, even if only at sub-conscious level, at some point in the future... .
Overall, it's mind-boggling