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“A reasonable amount of time” https://www.bloomberg.com/opinion/articles/2019-12-19/trader...

There have been cases where it happens after a certain amount of time after the announcement, technically, but where there wasn't enough time for speed of light delay to get the information to the place where the trade was executed.

https://www.washingtonpost.com/news/wonk/wp/2013/09/24/trade...

Hmm. In that case it didn't happen after the announcement. If the events have space-like separation, then neither 'before' nor 'after' are valid descriptions.

(And "simultaneously" is right out.)

Good luck telling regulators this, I suppose...

It's pretty vague. I recall guidance of waiting for two trading days, but that seems pretty conservative. Otoh, who wants to have to argue it with the SEC.

milliseconds, nanoseconds if you can

The moment the information is public.

You have to leave a reasonable amount of time to ensure that the information can be priced into the market.

The market's made of computers reading press releases, so that's well under a second.

That's just the market's "gut reaction" so to speak.