Nur wenn gefragt wird
Talking about ex-partners makes sense only when the new partner asks questions, wants to know about our past relationships, and provided it is not an obsession to him.
When talking about our past, not only the story is important, but also the way we tell it, because the way we talk about our past tells a lot about our present: are we still wounded, angry, emotionally abused? Or is our past behind our shoulders and we've moved on, we're ready for someone new?
When a partner talks often and spontaneously about his past relationships without being requested to, his wounds are still open and bleeding.
We have a choice: either accept them and take care of them (but there's no guarantee that we can heal them: we can only facilitate a process when it is already happening, we cannot trigger the process from scratch) or we can quit in order to avoid being emotionally challenged in turn and for an indefinite time that might even tend to "forever". Listening is important, in order to evaluate the situation and decide what to do.
I tend to quit when someone talks about past relationships without being requested to: I have moved on and I want someone as resilient and strong as me, a peer.
Unresolved past issues are food for psychiatrists and psychologists, not for a healthy relationship.