[deleted]

As usual ;-) I doubt it was only »a few days«.
However, even it was weeks, it still hard to explain how it’s possible that he arrived at Casterly Rock practically at the same time as the Unsullied, although he stopped at King’s Landing.

The only explanation that comes to my mind is that Euron himself wasn’t aboard the fleet that arrived at CR. Maybe he took off on a single ship after fighting Yara to bring his prosoners to KL, while the fleet continued to go to its next destination without him.
But that seems unlikely again – Euron missing a fight? Trusting his people to go to war without his leadership? I can’t imagine that.

Maybe it’ll be explained next episode. But I guess it’ll be just one of the tradeoffs we have to accepts because of the »fast-forward« mode the show has jumped into.

If Euron really has 1000 ships, he doesn't need all of them in one place. He probably left part of his fleet to defend Iron Islands in case of necessity, and Casterly Rock is close by. I presume that Euron himself didn't take part in the battle as he is on the opposite side of Westeros at the moment.