It seems The Batman film might not be part of the DCEU at all.
The Batman solo film has been the subject of plentiful rumors, and the latest one implies that the film will not be connected to the cinematic continuity at all (via Batman On Film). Now, this is just a rumor at the moment, but if true The Batman would join the Joker and Harley Elseworlds-like project that exists on its own.
If that pans out it would also mean director Matt Reeves would have the option to either use Ben Affleck as the Dark Knight or go a different route completely, though that doesn't necessarily mean he would switch who's under the cowl.
Even if he does, a second rumor suggests that Ben Affleck's Batman would have a send-off in either Suicide Squad 2 or Flashpoint. Suicide Squad 2 will start shooting in 2018, and is much more likely to happen than Flashpoint, though the rumor suggests that the latter of those actually has a part already defined for him.
Again, none of this is confirmed, but neither is outside the realm of possibility. Affleck has expressed nothing but excitement over working with Reeves on the Batman solo film, a project that Affleck was originally slated to direct. After stepping off the project as director (but staying on as its star), Reeves was brought in and decided to start from scratch.
Making the film stand on its own could free up Reeves to tackle any version of the Dark Knight he chooses, and that is a tempting offer for any director. You could also keep Affleck on and still reinvent the character, but he also might favor a clean slate as it were.
Right now it is unknown when fans will see Affleck again in the cape and cowl. Justice League 2 currently has no release date, and The Batman is also in flux. He could certainly show up in Aquaman or Wonder Woman 2 in a cameo fashion, but that remains to be seen.
In the meantime, fans can catch him in Justice League, which currently holds a 60.76 on ComicBook.com's composite rankings.
Justice League is in theaters now.