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.

I’ve said that in the modern world. It means how ever long the author knows it to be. Could be different with every story.

It indicates that it's a fairytale, so basically it happened in a whole different timeline.

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.