In a fiction story if someone says once upon a time a long time ago - how many years ago do you think?

Many children's stories beginning that way have medieval elements, though some are based on ancient myths/stories, Æsop's fables, etc. For those, ca. 12th cent. - 15th cent.

Some were written down by the Elizabethans (16th cent), others by those early in this country (17th / 18th centuries), in the time of the brothers Grimm or Dickens (early - mid 19th cent)...

So I assume what the descriptions, terminology, etc., suggest.

Depends on context but if it were out of the blue, I’d say 50+ years

100s, 1000s , I don't care about those things in fiction.

200 years without any context.

At least 100 years ago.