Alternative to "since when" that asks about time instead of expressing sarcasm, annoyment or doubt [closed]

I can think of a few ways.

The simplest:

When was El Cantar de mio Cid first considered a national epic of Spain?

(One could say, in lieu of "considered", "recognized" or "declared")

A little more wordy:

When did El Cantar de mio Cid first start to be considered a national epic of Spain?

Another option would be

How long has El Cantar de mio Cid been considered a national epic of Spain?

This has the advantage of sounding natural and being clear that you are discussing an ongoing (and accepted) status. The potential disadvantage is that you may get answers of the form "for [number] years" rather than "since [date]", and although these ought to be equivalent, in practice the former may be less precise.

Another way would be:

From when was El Cantar de mio Cid considered a national epic of Spain?