Try and talk you way out of this lore team challenge!

[Koujiro]

Now here's something interesting. Just how long did Nidhogg wait to get his revenge on the knights and King Thordan? We know that there were only four survivors from among the Knights Twelve, and presumably only offspring produced AFTER consuming Ratotskr's eyes would be imbued with dragon essence, right?

So, either all the Ishgardians are descendents of those four knights, or Nidhogg gave the King and the Twelve plenty of time to pump out some kids before going medieval on them...

The thing about Nidhogg is that he doesn't want to avenge Ratatotskr (at least not any more). He wants to make all Ishgardians suffer. Not only because they are all descendent from the Knights Twelve, but also because he doesn't trust - and even hates - humans any more (remember, the Elezen were the ones that broke the trust with the Dragons by foolishly listening to the Ascians and killing Ratatotskr for her eyes). And he's doing so for the whole duration of the Dragonsong War (a thousand years or so). That's why he never tried to finish the war. He simply tries to make them suffer as much as possible, for as long as possible. So he never "waited" to get revenge. The Dragonsong War itself is his revenge.