I think there are at least 3 chapters left.
The whole Circus thing will get an end this chapter. After that we will get a chapter about Allen and Mana travelling + some new Infos. The third chapter could be about Mana's "death".
I hope we will get around ~35 pages
Well, I'm not sure that Hoshino will reveal most important part of flashback right now, especially case Allen don't remember it, so it should be showed from Cross or Road perspective. But then again chapters 233-235 we saw as from Cross Marian. So it depends on Hoshino desires...
I mean she made chapter 234 just to hype whole flashback....and it worked. Case again it turn tables and after chapter 234 we finally got confirmation, that story is not about defeating Earl.
So apparently before that chapter people could assume how manga will end, and it always depended on surviving MC. Nothing else would change. And really rare aumont of people tried to prove that Earl is not evil. So chapter 234 basically just show how things are truly complicated. So yes, Hoshino revived D GrayMan, and even people who putted manga on pause returned case that particular chapter.
Ofcourse Hoshino knew about that it will work like this. And now she is holding possibility to show Mana's death or just end flashback and move to more important things. So flashback could end next chapter or after one year(or rather bext year)