Well, holdonasecond. The only number in the show that we ever got is that Nightmare Moon got bombed one thousand years ago. We don't actually know for how long did Celestia, Luna, and Discord live before that.

They're immortal beings?

And discord ruled UNTIL the sisters found the elements of harmony, and discord is cocky but he's invincible without the elements, so he probably ruled for a long, LONG, time until the sisters gathered the elements.

And then AFTER there was attack of Tyrek, which both Luna and Celestia were present for, as well as Star Swirl.

AND THEN, theres Sombra, who was defeated "A thousand years ago."

All three were major occurances that were just labeled as "A long time ago" or "A thousand years ago", but ALL occured before Luna was banished as she was present for them.

To say these major occurances, the usurping of a nation, the attack and banishment of a warlock, and the destruction of a nation, all occurred within a short timeframe, which in immortal years is a few centuries, of one another is pretty unlikely.