How long do I have to get the ashe world skin?

You have to get 57,3cm long

http://na.leagueoflegends.com/en/featured/worlds-hits-the-rift-2017/#loot

This skin will return to the store every year during Worlds, but the icon and loading screen border will only be available this year. 2017 is also your best chance to unlock her Golden Chroma and Ward, which will become legacy content following the tournament; note that you’ll need to either complete Worlds Master Pass missions or pick up extra tokens from Hextech Chests bundles from Oct 24th - Nov 5th to hit 320 Worlds Tokens.

Championship Ashe will be available in the store for 1,350 RP from September 18 until the day after Finals.

The finals are on the 4th which means you have until the 5th to get the skin until next worlds.