DeBlasio is currently engulfed in a series of scandals at city hall and trying to save his own ass next year in hopes to get re-elected.
I highly doubt any word on a new stadium will come out until after next years mayoral election in November -- at the earliest.
DeBlasio isn't going to back a stadium announcement now, because the spotlight is too bright on his scandals. And in the leadup to the election next year, approving a $300M stadium for a soccer team with dwindling attendance is not going to get the public enthused.
So I figure the timeline is:
2018 onward - stadium announcement
2020 onward - move into new stadium
That means at least 4 more seasons at Yankee Stadium.