In North America, it will be from October 9th to October 23rd. There may be a second event coming later for Marshadow.
The Pokemon website states, "This first opportunity to get Marshadow won’t last long. If you want to get this amazing Pokémon, be sure to visit GameStop from October 9–23. Then get ready to shine in battle with the Mythical Pokémon Marshadow in Pokémon Sun and Pokémon Moon!"
Sources: Pokemon Sun and Moon website