In the USA it is March 6th to April 10th.
In Europe it is March 4th to April 22nd.
So USA gets it for 5 weeks, UK/Europe for 7 weeks!
Also remember that you only need to download the Libery Pass in that time, you don't need to capture Victini right away. So you can buy the game on April 9th, get the ticket and wait several weeks until you get to the place you can use the ticket.