In Game Store

The original announcement said today (22nd) though a time wasn't given. Gamepress has it expiring in about 6.5 hours, but I don't know how accurate that is.

https://pokemongo.gamepress.gg

The home page has a timer that says four and a half hours. Don't know how accurate that is. But it is hours, not days, so don't delay if you want a box.

(Didn't read the previous answer before posting. Funny we both used the same phrasing for accuracy.)