Massively depends on context.
The date who spent most of the time talking about his girlfriend who died... I suggested he takes more time to grieve before trying to find a new girlfriend.
I have exes who are still friends, and I rent off one of them, so I have every expectation of mentioning him from time to time,and new man meeting him at some point. I definitely don't talk about him all the time though.
I'd expect to talk about past history enough that we've an idea of each other's past significant relationships (and why they ended.)
If there are children involved, and finances or other assets involved, it will be inevitable that they are mentioned from time to time.
If they sound really bitter when they talk about their ex, I'd be wary of that, and more so if a whole string of them all turned out to be psychos. That would suggest they either make poor choices (and if so, what does it say about me?) or more likely, never learn anything about their own poor behaviour.
If they're constantly mentioning that, "Sam used to do this really funny thing..." "Chris loved this thing where..." "This always makes me think of my ex," that would suggest they haven't moved on far enough, and that would be mentioning them too much.