Going by this information:
Pokemon D/P: Sep. 2006/Apr. 2007
Official Shaymin Reveal: Feb. 2008
Pokemon B/W: Sep. 2010/Apr. 2011
Official Meloetta Reveal: Mar. 2012
Pokemon X/Y: Oct. 2013
Official Hoopa Reveal: Feb. 2015
See the pattern of about a year and a half between? That means we should get it in about a year or so.
And this is assuming Marshadow will follow the shadow (ha, get it) of the Pokemon mentioned above.
If it will be more like Arceus/Genesect/Volcanion, we'll get it in about two years.