Writing

I would think a flashback that long risks causing readers to lose sight f the primary story line, not only interrupting but replacing the original story. As someone who frequently gets lost when a narrative refers to new characters or events out of blue, I can appreciate the use of flashback to give some background and context.

The question I would have is the time frame for the character having the flashback. Is the character old and musing about his or her whole life, in which case a long flashback might be appropriate. For example, an aging grandparent is writing a final letter to a grandchild and sharing key moments from his or her life.

Or is the flashback meant to be short, as when a piece of music triggers a quick memory of a past event, as when a song you haven't heard in years provokes a memory of your wedding day.

Have you thought about using a startling event in the character’s current time that could force the flashback to end or is the flashback needed to explain the upcoming actions of the character and others around her or him?

You have a really interesting conundrum. Let us know how it turns out!